If you are looking for the killer ap, you obviously havent played Splinter Cell.
I can understand if Halo is not your kind of game, but Splinter Cell is a masterpiece. If you can wait it'll be out for PC soon, but frankly I couldn't. The game is so incredible in every aspect I can't do it justice in a small post, but I encourage you to read pretty much any review (Gamespot, IGN, Firingsquad, etc.) and they summarize it pretty well.
Mechassault is another killer game. It balances mech sim with arcade style beautifully into an action rampage that is simply pure fun, in the way that Serious Sam and Shinobi were. And XBox Live multiplayer is a dream. I thought that deathmatch was as dead and stale as it could be, but Mechassault delivers in spades to completely revitalize the deathmatch experience.
I have not had a chance to pick up Steel Battalion ($200 controll and all) but I am hearing good things.
So, there are just 3 of many more titles that highly justify the console. I would suggest the PS2 as a first console, but if you had to choose between Gamecube and XBox, theres not much contest (but then I haven't had the chance to try Resident Evil 0 or the incredulously hyped Metroid Prime, of which I may rent one simply to play).
And if you think Splinter Cell and Mechassault suck, well then, check your gaming badge at the door and go play some DOOM or The Sims because j00 r0x0r 3Nd R 1337!
-DSR
"Diatribes are not my specialty. Getting pissed off is my specialty. Diatribes just come naturally from that."