what to upgrade first?

[David198502]

hey guys!
i have a pretty crappy rig and want to upgrade for COD to get playable framerates.but i have no idea about computers so i would appreciate suggestions on what do you think i should upgrade first.i know many would suggest to buy everything new,but my budget is limited.which upgrade do you think would have the most recognizable effect?
my systemspecs:
-Mainboard S1156 INTEL DH55TC,Micro-ATX,bulk
-Prozessor INTEL Core i5-650,3,2GHz,LGA1156,Dual-Core,Boxed
-2x 2GB DDR3 RAM,1333MHz,CL9,KINGSTON ValueRAM
-PCI-E 2.1 SAPPHIRE Radeon HD5570,Lite Retail,Low Profile
-Windows 7Home Premium 64Bit