In my opinion, keep the one that has the most games you are looking forward to, for me this is easily Xbox with games like Halo 2, Jade Empire, Conker, Outrun 2, Star Wars KOTOR II, MechAssault 2, Kameo, Forza, Republic Commando, Unreal Championship 2, DOA Ultimate, Battlefield Modern Combat, Doom 3 etc etc, not to mention the best version of all the multiplatform games.
I will say however you pretty much can't make a "bad" decision here. All 3 consoles do and will continue to kick ass and all have plenty of kickass games with plenty of kickass games coming. But if push comes to shove IMO nothing can top the Halo 2 -- Conker one two knockout combo... leavin fools like Jones Jr. sleepin on tha mat.
(I take that back, the GT4 -- Wanda Collossus combo on PS2 is just as lethal). Gamecube is probably the weakest, so you really only need to chose between PS2 and Xbox. Tough one, but I think Xbox has it.