Thanks to this GTAForums topic for the info.
* Like the upcoming PC version and even the PS2 version, the Xbox version has almost no loading times other than the beginning after you load a game.
* The framerate was noticably more stable than the PS2 version when the Dutch Xbox Magazine guys saw the game.
* The Xbox version will feature sharper textures than the PS2. Whether or not it will have the highest resolution textures that the PC will have it yet to be seen.
* The Xbox will feature a reworked lighting system rather than keep the PS2's version.
* It's no surprise that custom soundtracks are back, so you can put in your own music CD's and play them in-game on your own radio station.
* The Xbox version will feature a replay mode just like previous PC versions.
* The magazine mentioned that the two player cooperative mode will return in the Xbox version.
Secondly we have Italian magazine GMC - Giochi per il mio computer with a PC preview, no new information although they mention something about widescreen being available, GTA3 and VC had this so no suprise there. There are also some screens in the magazine.
Thanks to GTA-Series and this GTAForums topic for info/scans.
Finally German magazine Offizielles XBox Magazin has information and screenshots on the Xbox version. The Xbox version will apparently be more of a port of the PC version, rather than the PS2 version as GTA3 and VC were. Information from the magazine includes:
Thanks to GTASanAndreas.net and GTA-SanAndreas.de for the info, this GTAForums topic contains scans
* A replay mode will exist, just like in the PC version, allowing you to replay the last 30 seconds of your game, save the replay, and play it later.
* The two player mode will exist for the Xbox. (not for the PC however)
* The textures will be of higher quality.
* The number of polygons objects are composed of has been greatly increased.
* With the right ministick you will be able to spin the camera around your character or vehicle.
* It will be possible to have an MP3 radio station in the Xbox version, similarly to the PC version.