Definitely get Rogue Squadron II. It's one of my very favorite games, ever. If you get RSIII, you will only be able to play the RSII portion in co-op mode, not in single player. The single player is much better in RSII in my opinion. Still, RSIII has some good flight stages, and as everyone said, most of the other type of missions were not so hot. I did enjoy the AT-ST stage though.
Some other good third party GC games(you should know the first party ones)...
REmake
RE0
Viewtiful Joe/Viewtiful Joe 2
Killer 7(though it's not for everyone)
Prince of Persia: Sands of Time
Soul Calibur II
Tales of Symphonia
Skies of Arcadia
Super Monkey Ball/Super Monkey Ball 2
Eternal Darkness(was a second party effort)
F-Zero(made by Sega)
Pikmin/Pikmin 2(Nintendo games, but often over-looked)
"I feel like I could take on the whole Empire myself."
