I used to have a c2d e6600 o/c to 3.0 with a gtx275 and 3gigs of memory.
GTAIV ran horribly at only 8-19 fps on no matter what setting I put it on. It even ran like that on 800x600 with lowest settings which made absolutely no sense. What I did find out is that it always maxed out the cpu at any resolution.
Anyway, I recently bought a core i7 920 with 6 gigs of ram and the same gtx 275. I now run it with an average of 50 fps with the highest being 60fps (due to v-sync) and lowest being around 40fps (when it's foggy, raining, and lots of traffic. However, most of the time its really stable at around 50fps.
It's amazing what just the 920 did for frames. I don't even think I was limited by memory, even with only 3 gigs as the benchmark would always say I plenty of system memory left over. Of course, the game is more demanding the benchmark, but still, I think the bottleneck was defiantly the cpu.
This game was defiantly not a very good port and the gameplay is just, alright. The story is good and I do like watching the cutscenes but the gameplay is not as good a San Andreas. There is no role playing aspects that I liked with San Andreas and the world is just as dull as with any of the other games.
Money is pretty much pointless in the game and the only way to spend it is to go on shooting sprees and buy a ton of ammo. Even then, I always have more then enough money.
Next game I hope they add the ability to purchase houses, buildings, businesses, cars, boats, campers?, and actually have some kind of economy and A.I. that doesn't reset itself every few minutes. I would really like to cause havoc in the middle of the city and actually cause havoc instead of everything magically going back to normal once I leave.