I myself play only one game on the PC online - Everquest (great RPG!).
I have an Xbox, and think it is well worth getting an Xbox. XBConnect (free Xbox Live service) and the brandspanking new Halo 2. Good graphics, good play, good fun.
However, the only disadvantage with Xbox play is to host a game, you need a decent upload speed - preferable 128k upload per xbox joined - I can't host a 1v1 game on NTL as the upload speed caps at around 70kbs.
If your just looking to play, go Xbox (unsure about PS2, but I hear it's crummy). If you want to host games, and inteded playing games like Counterstrike or Rainbow 6, go PC.