Advice for computer upgrade!

[slojani]

I'm not a computer expert so please for the smart advice on upgrading your computer with the new graphics card in my computer's current configuration, that is:
Asus P5N 32 E-SLI
Processor Intel Core2 Duo E8400 overclocked to 3690Mhz
8GB DDR2 memory
Graphics Card HD5670 GDDR5 Shapiro 1MB
Windows 7 64bit
display 3 x 23 "ACER A231H Eyefinity setup 2328/1280 display

When playing IL-2 in 1946, this system is sufficient. IL-2 play CoD with this system is not good-bad FPS and minimal graphics.
Please if you can recommend cost-effective upgrade.

Thank you for advice !