Radeon Cucore

OS: Windows 7 Ultimate 32-bit
Processor: AMD Phenom(tm) II X4 940 Processor (4 CPUs), ~3.2GHz
Memory: 4096MB RAM
DirectX Version: DirectX 11
GFX: ATI Radeon HD 5700 Series (ASUS EAH5770 CuCore)

Other

Motherboard: M3A78 PRO (socket AM2, DDR2 RAM)
PSU: Fatal1ty 550w
HDD: Western Digital 500GB + Seagate 1TB (both SATA3)
Case: CoolerMaster 690
RAM: 2 x 2GB 1066MHz DDR2 (not sure about brand)

I run games at 1920x1080 @ 60Hz, but ALWAYS disable AA and most high-res shadows and shaders, occasionally reducing draw distance as well.
I don't want to go too insane with CrossFire 6990s, but something that will definitely run Battlefield 3 (and newer games) at 60+ FPS.

My main concerns with upgrading specific components is whether the PSU will handle it, having to change Motherboard to accommodate DDR3, different CPU etc. So I can understand if I can't upgrade without an entire overhaul.

Anyway thankyou for suggestions!