The X-Box titles will be exactly the same, except for slightly less polygons being pushed around.
GC exclusive games :-
Eternal Darkness - Resi Evil type game, very very good. I prefer Resi Evil personally, but many think this is better, certainly the controls are a little more polished.
F-Zero - One of the finest racers ever made.
Super Monkey Ball - Should be able to pick this up cheap. The first is better in single player mode, the second is better for multiplayer.
Ikaruga - One of the best shooters ever made.
Mario Golf - Good mulitplayer fun (you may prefer Toger Woods if you like serious golf games)
Super Mario Sunshine - Good Mario game.
Games coming soon :-
Rogue Leader 3 - This looks spectacular, I am waiting for the first reviews before I decide to buy.
Mario Kart Double Dash - This is going to be awesome.
Viewtiful Joe - Will be interesting my copy should arrive any day soon, it is a side scrolling beat-em-up though, so may not appeal.
Final Fantasy chronicles - The first FF game to grace a Nintendo home console for years. If FF Tactics on GBA is anything to go by, this will be something special.
1080 Avalanche - Hopefully as good as the first
Metal Gear : Twin Snakes - Remake of the original NES classic.
Resi Evil 4 - Needs no intro