First open the sim up and go to Hardware Setup>Video Options and set everything to Excellent or Perfect.
Next go to your config file at :C:\Program Files\Ubisoft\IL-2 Sturmovik 1946\conf.
It will look something like this:
[il2]
title=Il2-Sturmovik Forgotten Battles
hotkeys=HotKey game
[window]
width=1920
height=1200
ColourBits=32
DepthBits=24
StencilBits=8
ChangeScreenRes=1
FullScreen=1
DrawIfNotFocused=0
EnableResize=0
EnableClose=1
SaveAspect=0
Use3Renders=0
[GLPROVIDER]
GL=Opengl32.dll
[GLPROVIDERS]
Open GL=Opengl32.dll
DirectX=dx8wrap.dll
[NET]
speed=8000
routeChannels=0
serverChannels=15
localPort=21000
remotePort=21000
SkinDownload=1
serverName=No Name
serverDescription=
remoteHost=
localHost=91.255.147.97
socksHost=
checkServerTimeSpeed=1
checkClientTimeSpeed=1
checkTimeSpeedDifferense=1.5
checkTimeSpeedInterval=2
checkRuntime=1
[MaxLag]
farMaxLagTime=5.0
nearMaxLagTime=2.0
cheaterWarningDelay=3.0
cheaterWarningNum=-4
[chat]
region=(dx=0.6925,dy=0.071666665,x=0.0,y=0.0)
adr0=ALL
msg4=47s\u0020beat\u0020feet\u0020taKE\u0020OUT\u0 020THE\u0020FLAK...
[game]
Arcade=0
HighGore=1
mapPadX=0.66875
mapPadY=-0.046666667
viewSet=33
Intro=0
NoSubTitles=1
NoChatter=0
NoHudLog=0
NoLensFlare=1
iconTypes=2
eventlog=eventlog.lst
eventlogkeep=1
3dgunners=0
SubTitlesLines=4
ClearCache=0
TypeClouds=1
[HotKey game]
PrintScreen=ScreenShot
P=pause
Pause=pause
[HotKey gui]
Escape=activate
[HookViewFly Config]
timeFirstStep=2.0
deltaZ=10.0
[HookView]
MouseLeft=Len
[HookView Config]
AzimutSpeed=0.1
TangageSpeed=0.1
LenSpeed=1.0
MinLen=.50
DefaultLen=20.0
MaxLen=30000.0
Speed=6
[HotKey builder]
MouseLeft=objectMove
MouseRight=popupmenu
Enter=freeView
Shift MouseLeft=worldZoom
Alt MouseLeft=select+
Alt MouseRight=select-
Alt Ctrl=unselect
PageDown=change+
PageUp=change-
End=change++
Home=change--
Ctrl MouseLeft=insert+
Insert=insert+
NumPad-0=insert+
F=fill
Ctrl MouseRight=delete+
NumPad.=delete+
Delete=delete+
Backspace=cursor
Tab=cursor
F10=land
F11=onLand
NumPad-=normalLand
NumPad+=toLand
NumPad-5=resetAngles
NumPad-8=resetTangage90
NumPad-7=stepAzimut-5
NumPad-4=stepAzimut-15
NumPad-1=stepAzimut-30
NumPad-9=stepAzimut5
NumPad-6=stepAzimut15
NumPad-3=stepAzimut30
[MouseXYZ]
F1=SpeedSlow
F2=SpeedNormal
F3=SpeedFast
MouseRight=XYmove
F4 MouseRight=Zmove
MouseMiddle=Amove
F5 MouseRight=Amove
F6 MouseRight=Tmove
F7 MouseRight=Kmove
[MouseXYZ Config]
RealTime=1
[HotKey Console]
Shift Tab=Activate
[Console]
HISTORY=1024
HISTORYCMD=1024
LOAD=console.cmd
SAVE=console.cmd
LOG=0
LOGTIME=0
LOGFILE=log.lst
LOGKEEP=0
[sound]
SoundUse=1
SoundEngine=1
Speakers=4
Placement=3
SoundFlags.reversestereo=0
RadioFlags.Enabled=0
RadioEngine=2
MusicVolume=8
ObjectVolume=14
MusState.takeoff=0
MusState.inflight=0
MusState.crash=0
MusFlags.play=0
MasterVolume=13
Attenuation=5
SoundMode=1
SamplingRate=2
NumChannels=2
SoundExt.occlusions=0
SoundFlags.hardware=0
SoundFlags.streams=1
SoundFlags.duplex=1
SoundExt.acoustics=0
SoundExt.volumefx=0
SoundFlags.voicemgr=0
SoundFlags.static=1
VoiceVolume=7
Channels=2
SoundExt.extrender=0
SoundFlags.bugscorrect=0
SoundSetupId=8
ActivationLevel=0.02
Preemphasis=0.85
RadioLatency=0.5
AGC=1
PTTMode=1
SoundFlags.UseRadioChatter=0
SoundFlags.AutoActivation=0
RadioFlags.PTTMode=0
RadioFlags.PlayClicks=0
ActLevel=0
MicLevel=0
SoundFlags.forceEAX1=0
[rts]
ProcessAffinityMask=1
mouseUse=2
joyUse=1
trackIRUse=1
DisableIME=0
locale=
[rts_mouse]
SensitivityX=1.0
SensitivityY=1.0
Invert=0
SensitivityZ=1.0
[rts_joystick]
X=0 1 4 9 16 25 36 49 64 81 100 0
Y=0 1 4 9 16 25 36 49 64 81 100 0
Z=0 10 20 30 40 50 60 70 80 90 0 100
RZ=0 10 20 30 40 50 60 70 80 90 100 0
FF=0
U=0 10 20 30 40 50 60 70 80 90 100 0
V=0 10 20 30 40 50 60 70 80 90 100 0
1X=0 53 54 56 59 62 66 71 77 83 90 0
1Y=2 55 57 59 62 66 71 77 84 91 100 10
1RZ=0 100 100 100 100 100 100 100 100 100 100 0
1U=0 100 100 100 100 100 100 100 100 100 100 0
1V=0 100 100 100 100 100 100 100 100 100 100 0
1X1=0 100 100 100 100 100 100 100 100 100 100 0
1Y1=0 100 100 100 100 100 100 100 100 100 100 0
1Z1=0 100 100 100 100 100 100 100 100 100 100 0
1X2=50 0 11 22 33 44 56 67 78 89 100 0
1Y2=60 100 100 100 100 100 100 100 100 100 100 0
1RX2=0 100 100 100 100 100 100 100 100 100 100 0
1RZ2=0 100 100 100 100 100 100 100 100 100 100 0
1U2=0 100 100 100 100 100 100 100 100 100 100 0
1V2=0 100 100 100 100 100 100 100 100 100 100 0
1RX1=0 100 100 100 100 100 100 100 100 100 100 0
1RZ1=0 100 100 100 100 100 100 100 100 100 100 0
1U1=0 100 100 100 100 100 100 100 100 100 100 0
1V1=0 100 100 100 100 100 100 100 100 100 100 0
[Render_DirectX]
TexQual=3
TexMipFilter=2
TexCompress=0
TexFlags.UseDither=1
TexFlags.UseAlpha=0
TexFlags.UseIndex=0
TexFlags.PolygonStipple=1
TexFlags.UseClampedSprites=0
TexFlags.DrawLandByTriangles=1
TexFlags.UseVertexArrays=1
TexFlags.DisableAPIExtensions=0
TexFlags.ARBMultitextureExt=1
TexFlags.TexEnvCombineExt=1
TexFlags.SecondaryColorExt=1
TexFlags.VertexArrayExt=1
TexFlags.ClipHintExt=0
TexFlags.UsePaletteExt=0
TexFlags.TexAnisotropicExt=0
TexFlags.TexCompressARBExt=1
TexFlags.TexEnvCombine4NV=0
TexFlags.TexEnvCombineDot3=0
TexFlags.DepthClampNV=0
TexFlags.SeparateSpecular=0
TexFlags.TextureShaderNV=0
HardwareShaders=0
Shadows=2
Specular=2
SpecularLight=2
DiffuseLight=2
DynamicalLights=1
MeshDetail=2
VisibilityDistance=3
Sky=2
Forest=3
LandShading=3
LandDetails=2
LandGeom=2
TexLarge=1
TexLandQual=3
TexLandLarge=1
VideoSetupId=4
ForceShaders1x=0
PolygonOffsetFactor=-0.15
PolygonOffsetUnits=-3.0
[Render_OpenGL]
TexQual=3
TexMipFilter=3
TexCompress=0
TexFlags.UseDither=1
TexFlags.UseAlpha=0
TexFlags.UseIndex=0
TexFlags.PolygonStipple=1
TexFlags.UseClampedSprites=0
TexFlags.DrawLandByTriangles=1
TexFlags.UseVertexArrays=1
TexFlags.DisableAPIExtensions=0
TexFlags.ARBMultitextureExt=1
TexFlags.TexEnvCombineExt=1
TexFlags.SecondaryColorExt=1
TexFlags.VertexArrayExt=1
TexFlags.ClipHintExt=0
TexFlags.UsePaletteExt=0
TexFlags.TexAnisotropicExt=1
TexFlags.TexCompressARBExt=0
TexFlags.TexEnvCombine4NV=1
TexFlags.TexEnvCombineDot3=1
TexFlags.DepthClampNV=1
TexFlags.SeparateSpecular=1
TexFlags.TextureShaderNV=1
HardwareShaders=1
Shadows=2
Specular=2
SpecularLight=2
DiffuseLight=2
DynamicalLights=1
MeshDetail=2
VisibilityDistance=3
Sky=2
Forest=2
LandShading=3
LandDetails=2
LandGeom=2
TexLarge=1
TexLandQual=3
TexLandLarge=1
VideoSetupId=17
Water=3
Effects=1
ForceShaders1x=0
PolygonOffsetFactor=-0.15
PolygonOffsetUnits=-3.0
tUnits=-3.0
Units=-3.0
[DGen]
NoBadWeather=1
CampaignLength=Medium
Maxbomberskill=0
RandomFlights=1
MaxFLAK=5
UseParkedPlanes=1
GroundIntensity=Low
AirIntensity=Low
MissionDistance=90
HistoricalRanks=1
NoAirfieldHighlight=1
CampaignVictory=100
SlowFire=1.0
[quick]
Custom0=NWEurope
Custom1=MTO
Custom2=Bessarabia
Custom3=Net5Summer
Custom4=Guadalcanal
Custom5=Sardinia
Custom6=Murmansk_Winter
Custom7=fly_Z_Britain_Autumn
Custom8=Midway
Custom9=The_Slot_1-44
Custom10=V1_Summer_1944
Custom11=zip_Alpen
Custom12=Mbug_Slovakia_winter
Custom13=dlv_Sinai
Custom14=BoB
Custom15=Burma
Custom16=Ardennes_W
Custom17=Channel
Custom18=Hawaii
Custom19=Kurland_Autumn_online
Custom20=Kuban
Custom21=Norway
Custom22=NewGuinea
Custom23=NWEurope
Custom24=OnlineITaly
Custom25=Singapore
Custom26=SandsOfTime
Custom27=Stalingrad_Winter
Custom28=vol_ItalyAfricaGreece
Custom29=Berlin
Custom30=Balaton
Custom31=Normandy2
Custom32=Channel
[Mods]
BombBayDoors=0
SpeedbarTAS=0
SeparateGearUpDown=0
SeparateHookUpDown=0
SeparateRadiatorOpenClose=1
ToggleMusic=0
AirShowSmoke=2
Scroll down to the open GL portion of the setup (I am assuming that you have switched to Open GL from DX as previously suggested) and find the line water... set it to 1. Try the sim on a map with water.. if you like that and you get decetn frames.. then try water 2... if it still runs well try water 3. I am not that familiar with your video card.. The first step is to set things to perfect in the sim though. Play around with it....
Also take a look at:
Config Guide @ M4T
Optimisation Guide @ M4T
IL2 specific link @ M4T
Hope that helps.. and welcome aboard.