Performance wise, the game is very hit-and-miss. Some people with pretty average PCs could run it at launch whereas even today some decent rigs struggle with it.
I think people are spoiled by high minimum framerates, and don't see how crap it looks and how it runs on ps3 and xbox360.
The graphics settings on consoles:
Resolution 1280 x 720 @ 20 -30 fps
Texture Quality Medium
Reflection Resolution Medium
Water Quality Medium
Shadow Quality High
Render Quality Low
View Distance 21
Detail Distance 10
Vehicle Density 33
Shadow Density does not exist on the Xbox 360/PS3
Definition off
Vsync off
If you play the game at those settings, a mere gtx260 coupled with a q6600 would run it great. Obviously you shouldn't play it with a dual core... I don't get why needing a faster cpu relatively for its time is a bad thing. People also complained about Supreme Commander, I for one am happy the game ( still talking about supcom) supports 12 threads as of this day, even though back in the day my Pentium D had trouble!
I have dated hardware now but have no problems playing gta IV with all the stuff maxed, yes there is a slight frame drop in some areas ( notably on the most eastern island), but nothing near as bad as on the consoles.
I have a 780 with an i5 at 4Ghz and that struggles with everything max.
Either your definition of struggling is different from mine, or something is off/malfunctioning. I play the game supersampled at 2560x1600 with everything maxed except traffic ( at 66) on just a gtx570, and it runs smootly.
EDIT: Fair enough, just double checked, it runs at 35 ish fps average, which isn't great, but it's enough for me.