wow! 200$! that is quite awesome you can get an honorable card with quite satisfying or even more performance the radeon hd 6870,GeForce GTX 560 would be awesome.But it wont give you it full throttle because your CPU and RAM are almost obsolute.To get max performance out of these cards you would require at least
i3-2120
ddr3 4 GB ram
we cant just change the CPU and memory only you should get a new motherboard for ur CPU and ram also the psu(power supply) will serve as a problem because the radeon hd 6870,GeForce GTX 560 are quite power hungry cards
This is not possible since it will overshoot ur budget of 200$
also just upgrading your gpu to latest and using poor CPU would not do much good in most games especially in sandbox games(GTA 4 etc)
so the most good choice would be going for sub 150$ cards or even sub 100$ card
The
Radeon HD 5670($75)
Radeon HD 6670($95)
would be nice
still if u wish to going to a higher card don't go to the
radeon hd 6870,GeForce GTX 560
Try
Radeon HD 5750/6750($100)
Radeon HD 6790($125)
Radeon HD 6850($150)
BUT REMEMBER THESE CARDS WONT PERFORM IN THEIR MAX THROTTLE
Hope this helped