I think the other reviewers here have already covered most of the relevant stuff about the Xbox version of GTA: San Andreas. I'd been playing the NTSC PS2 version since release, and was impressed enough to place my pre-order to "upgrade" to the Xbox version, so consider this as more of a comparison review.
The GTA series needs little introduction - it's the benchmark for free-roaming, go-anywhere games, and GTA:SA offers unparalleled freedom in a game-world 3 times the size of Vice City, adding almost RPG-like "consequence" elements similar to "Fable". Eat too many pizzas, you get fat. Work out, you get fit. Ride your bike a lot, you get better at it. Never change your clothes, you start to smell - and passers-by don't mind letting you know! Just like real life, eh?
I suppose the very fact that I was buying a game which I already owned on a different console is testament to how good this is, but when I pre-ordered it I was expecting the same leap in quality that was seen with "GTA III" and "Vice City", where the Xbox versions were far superior to the PS2 versions in terms of graphics and frame-rate. Strangely, considering "San Andreas" is far more ambitious than its predecessors, the difference is not so marked this time, and the Xbox version is actually a bit of a mixed-bag of better and worse.
60Hz mode is supported (as with Xbox GTA3 and VC) which is always a good thing, but graphically it's only slightly better. Pop-up is still prevalent, with trees in particular popping up worryingly close-by, but the overall look of the Xbox version is 'smoothed' where the PS2 version was 'grainy'. However, the Xbox version has some all-new graphical 'quirks' of its own, like flickering walls that can't decide whether they're in shadow or not, and tree branches that blend to sky-blue at the edges - fine if they're actually framed against the sky, just plain weird-looking if they're framed against other branches! And then there's the way the night sky flickers horribly between midnight and 2am... The PS2 version may look grainy and lo-fi compared to the Xbox version, but it didn't have these curious 'glitches', so it's hard to say that one is intrinsically 'better' than the other graphically.
The frame-rate is definitely higher on the Xbox but the draw distance isn't much different to the PS2, if at all, and it's still prone to going choppy at times. The biggest problem with the Xbox version though is the control layout. The "look-left, look-right, hold-both-to-look-behind" buttons, mapped to L2+R2 on PS2, are mapped to the black and white buttons on the Xbox controller because it lacks a second set of triggers. This means that what felt natural on a PS2 controller is now rendered nigh-on impossible for the human hand to do - especially looking behind when you're driving which requires you to mash your thumb across both buttons and hope for the best. As with the graphics, it's swings and roundabouts with the controls too, because the Xbox version boasts the ability to rotate the camera freely and smoothly with the right thumbstick at all times. Other Xbox additions include a 30-second instant replay function and the ability to use custom soundtracks from your own CDs - cool features which probably outweigh those graphical niggles and control issues.
All in all, it's the game that matters - and the bottom line is that GTA:SA is a superb game whatever format you buy it on. Both PS2 and Xbox versions have their own good and bad points so I'd say the two versions come out fairly equal, with a slight advantage going to the Xbox version - as long as you can live with those control issues. It's not that much better than the PS2 version, though, and certainly nowhere near enough to make your PS2-owning mates turn green if that's your aim. Hey, forget about the hardware, and just enjoy the software!
One last thing... As a white middle-class male, the incessant "hangin' wit' my homies in da hood" "urban" thing doesn't half grate! Sorry guys, I know it's supposed to sound all "hard" and "street" and stuff, but talking the way these guys do is every bit as much of an affectation putting on a camp voice as far as I'm concerned! It's all a bit Ali G for me, coming off like GTA: Staines or something. Anyone who doesn't speak fluent "street" might wish the subtitles weren't just verbatim - in fact, I really think Rockstar missed a cool trick here... Anyone remember the classic "I speak jive" scene from the movie "Airplane"? It would have been SO funny (and helpful!) to have an alternative subtitle track that translated what your homies lay on you into the Queen's English! "Thank you my friend, I wholeheartedly agree with you" Haha! Fo' Sho'.