Yesterday I got GTA IV from a friend of mine and I was just installing it an hour ago. When I started playing I lagged non stop and it took me 10 minutes just to be able to press escape and quit the game. I've never had any problems playing any 'Big' games (Crysis 2, Shogun 2 and so on) and this is the first time it lags, if you can call it lagging cause this is just watching one frame per minute.
I tried again after I quit the game after shutting all non essential programs in the background but it still lagged. I'm running a computer wider scan with AVG just in case I've got a bugger on my computer but I run it regularly so it probably won't find anything. I just bought my computer last autumn, and cost a lot:
Motherboard:ASRock P55 Deluxe Intel LGA1156
Processor: Core i7 860 Lynfield
Ram: 4GB Value PC3-10660
Graphics card: GeForce GTX 470
Depending how much it cost and my limited knowledge on computers my fair assumption is that it shouldn't lag.