I just got Xbox - Ive been a PS2 fan for a while. I got Xbox because i wanted all those xbox only racing titles like project gotham and midtown madness..
Other than that - if you already have PS2 - i wouldnt bother buying Xbox. As was said before, most of the 3rd party games (NFS underground, NFL 2kX, etc) are available for both systems. The only advantages Xbox has is its better graphics (personally, its not being used to its full potential - compare Project gotham's rendering of the '99-'04 Mustang to Gran turismo 3's rendering - you can see the difference. Looks like someone in project gotham crapped out on the modeling side..) and its built in HDD. One good feature is that you can copy cds to its HDD and, if the game supports it - replace the games soundtrack with your songs.
I bought the Xbox live kit but i havent had time to open it yet - but from what im told its miles above sony's online play. Sony's online play was pretty good - however it took forever for it to refresh the game lists and lag was still a huge problem (yes, i do have broadband).
So thats it - IMO PS2 does have the better games, but Xbox does have the better graphics. I myself am just keeping my PS2 to play the PS2-only games (basically only gran turismo 4 lol), and i plan to buy all the 3rd party games for xbox rather than ps2.