From what I've played graphical improvements include the following-
Framerate stays at 30 FPS 90% of the time instead of 50% of the time
80% of the textures have been upgraded to slightly higher quality
Draw distance is better
Obnoxious pop up isn't has common but does still happen(the kind where you hit a wall you literally couldn't see until you were right in front of it)
Other improvements-
The initial loadtime is like 10 seconds or so instead of nearly a full minute, loadtimes while playing the game are practically nonexistent despite the addition of an actual pointless loading screen for entering buildings. On the ps2 version spraying tags would cause a 2 second load to finish the job, xbox just does it instantly.
Custom soundtracks- so if you do get bored of the many songs in san andreas you can listen to your own music.
Replay feature- watch the previous 30 seconds of your game over again.
Negatives-
Apparently due to the addition of a 480P mode , when played on an HDTV the game looks kind of jaggtacular , this I cannot confirm however becuase I dont' have my own HDTV.
Control- most people would prolly consider the xbox gamepad controls worse then the dual shock, I happen to prefer it but ther eyou have been warned.