I doubt Core 2 Quad Q9550 and GF 8800GTX among other things would have any problems running GTA IV. (and in 6 months I'd probably have bought the newest Radeon, but gotta see how they evolve)
Right now I have a really good pc with 8800gt, phenom, 4gigs of fast ram and other nice stuff + a hd screen and I'm telling you that ps3 and even 360 kills it.
I played assasins creed and was like wow, but then went to my friends house and played it on 360 and it was much much better.
I think it depends on people but to me PC is not made for gaming. keyboard + mouse sucks and it's only good for fps games and strategies.
pc version of san andreas is not playable to me, while I love it on my xbox.
The only games I play on my pc rather than consoles are Counter-strike and C&Cs.
imo it's uncomparable - the way you feel the joystick and you do exactly what you want, analog buttons so you can move in any direction, speed up only as fast as you want etc.
and most of all you don't have to buy new stuff every few months, worry about fps, software etc.