I just loaded up San Andreas - managed to get a copy through a buddy whose dad was on a bizz trip in the UK.
I've been playing for about 30 minutes and I'm gobsmacked at the size of this game - if you think Vice City was big, just the ONE city you start in is about 3 times as big and the detail is far higher.
I've never seen the game on PS2, but from what I've read, the PC version kicks it's ass six-ways sunday in terms of detail.
It's not jaw dropping visuals like Half-Life2 or Doom3 that grab you, because the engine doesn't work that way, it's the attention to detail which makes you wonder how the hell they did it.
If you've got a serious rig - AMD 64 + a 6800 or eqiv, you'll be way impressed.
I cranked up the anti-aliasing to 4xQ on my lowly 6200 (amd 2400+) and it looked absolutely stunning, but was too choppy to play nicely.
Dialogue is taking a bit of getting used to for an old fart like me - all the "down in the hood" stuff, but it's done so well it's like playing a movie + the cut scenes are way better in terms of animation and content quality - it really sets the mood. It's like a Spike Lee Joint
Vehicle physics is also more realistic - I was so used to driving around Vice City, it came as a shock that things aren't as easy in San Andreas.
The traffic, for instance, doesn't "obey itself" like in VC - it is far more realistic, so you have to keep your eye on the road at all times.
I was cruising on a highway and pranged into the back of someone and struth g0d, I caused an 8 car pile up ! - the physics was done perfectly.
The main characters animations are so much more realistic and smooth ...
Heck, this could just be the best video game yet made ... but then, as a self-confessed GTA Junkie, I'm biased