I've just ordered a pc with these specs (not an expensive system, spent about 840$):
Processor: AMD PHENOM 9950 QUAD-CORE 125 AM2+ 4MB
Motherboard: GIGA-BYTE 770/SB600 SAM2 ATX/GA-MA770-DS3
RAM: 2 x A-DATA 1GB DDRII PC6400 DDR800 DIMM1GBPC800-RPPAK2
Hard Drive: SAMSUNG F1 750GB SATA2 7200RPM 32MB HD753LJ
Graphics: ASUS RADEON HD4850 PCIE 512MB GDDR3
I'm pretty sure GTA IV will run fine on this system, but the question is, can I play it maxed out, and if not, does anybody with a similar system or just a lot of experience with GTA IV know what are the optimum settings knowing that my main goal is not to ever let the performance drop under about 26 fps.