Whoa with the bad user reviews,
I don't have a super machine, but the game is relatively smooth, so the low ratings from people who have technical difficulties is surprising, accept when you consider that the minimum specs on the game require a dual core processor, which many people do not yet own, but bought the game anyway. So the criticisms are more from people who are DISAPPOINTED that the game is a next-gen dual-core game - I would be pissed too, but when you've been playing computer games as long as I have (30 years), there is always some game that will come out that forces you to wait until you get a new PC (can you say, Falcon 4.0!) Now GTAIV has set the bar for single core players to upgrade their rigs to the current state of the art in order to play (I understand the anger, but this will not be the last time. It's how the game industry drives computer sales.
Many are having problems with game crashes and poor performance because they missed the minimum requirements (It crashed on me too until I updated my computer with:
.NET 3.5
XP Service Pack 3
I have a dual core 2.1 ghz, 4gb ram, Nvidia 8800 GTS (320mb - not even 512mb) and it runs smooth.
To force decent graphics create a file called commandline.txt in main GTAiV directory and cut and paste the following:
-nomemrestrict -norestrictions -novblank -percentvidmem 100 -renderquality 4 -shadowdensity 16 -detailquality 99 -texturequality 1