Well i'd get the 9800GT 512MB
Here are the recomended requirements:
Recommended System Requirements
* OS: Windows Vista - Service Pack 1 / XP - Service Pack 3
* Processor: Intel Core 2 Quad 2.4Ghz, AMD Phenom X3 2.1Ghz
* Memory: 2 GB (Windows XP) 2.5 GB (Windows Vista)
* 18 GB Free Hard Drive Space
* Video Card: 512MB NVIDIA 8600 / 512MB ATI 3870
And plus 9800GT is faster than 9600GT in any game out there, it has double amount of shaders (64 vs 112) so 9800GT
I think both either card would run good on that PSU....
edit- you should also try OC'ing that CPU if you see a bottleneck coz GTA4 requires a quad to run smooth
you mean Iball 450 w PSU is enough to run 9600 Gt as well as 9800 gt?