I'd base my system choice on the types of games you like:

X Box: mostly sports and action games, but it does them very well. Only one with decent online play so far (though you have to pay for it and need broadband)

PS2: a little bit of everything, but mostly platformers like Ratchet and Clank and RPGS.
Free online play with an existing internet connection, however, not very enjoyable due to too many idiots with 56k modems.

GameCube: Mostly family-oriented games. Mario and Zelda series. Also has Resident Evil, if you like those games.
No online component, but most of the one console multiplayer games are great on it.
Some online play