I covered this game quite extensively in the PS2 review, and aesthetics aside, not much changes in the transition from Sony's system to the Xbox/PC.
Like the ports of GTA III and Vice City, the PC and Xbox versions of the game have significantly improved textures and support for higher resolutions than the PS2 original. Both versions also allow the player to create their own custom radio station with mp3 files from their computer or CD files ripped to the Xbox hard drive.
Both the Xbox and PC editions look great, especially when compared with the low-res PS2 game, but the unmatched power of the PC platform gives it the technical edge. Building your custom radio station is also much easier on the PC, given the Xbox's painfully slow CD ripper. This said, the PC version doesn't control nearly as well for the in-car portions, so that's something to keep in mind.
Overall, you can't go wrong no matter what platform you pick. GTA San Andreas is the same great game on any system. This port just polishes an already superb experience to nigh-perfection. Now, good luck finding a copy on shelves…
-George