Uhhhhhh...
Maybe you had windows 7 32 bits?
That video card shouldn't have problems with GTA, I have an ATi 4330 which is like the one that CWF has but with slightly lower clock speeds, and I'm pretty sure that I can run GTA IV >.>
Anyways you shouldn't change to 32 Bits, since 32 bit architecture doesn't support more than 2,5Gb of RAM (or 2GB, I don't remember), and you would be wasting 1.5 (or 2) Gigabytes of RAM, just like I'm doing right now