GTA4 needs at least a 2.4 Ghz dual core just to run.
If you want to actually PLAY the game usefully (above 18fps) you need to overclock that 2.4 closer to 3.0 Ghz, or buy a faster dual core and overclock that, or buy a quad core and live off beans and sunshine for a month.
It's the only game on sale at the moment that requires such gargantuan processing power. You can get away with a mediocre graphics card and memory, it's the shear scale of processing the environment that takes its toll. Irritatingly. Not that I'm bitter at all, not that it's forced me to max out a black edition Amd 7750 to over 3.0 ghz, oh no, nothing like that...
Probably not even relevant now but I have an asus with the 8300 nvidia chipset. Overclocking is not as easy as it would be with a 7-series chipset. Again, not bitter that I bought the wrong motherboard or anything... na, not worn out MY BLOODY FINGER PRINTS FROM HOLDING DOWN THE BLODDY OFF BUTTON SO MANY BLOODY TIMES.. no... certainly not