Before you sell it you might want to give Eternal Darkness, Viewtiful Joe, Ikaruga, Pikman, Super Smash Bros, F-Zero, Mario Golf, Super Monkey Ball, Animal Crossing and Final Fantasy Crystal Chronicles a go. All of these games are exclusive to the system and they are all spectacular. Resident Evil 4 and another Metroid are also right around the corner, so by the time you've played a couple of those games you might have a couple more reasons to hold off.
Admittedly there are a lot more good games for PS2 than Gamecube, and Xbox has intoxicating hacking/pirating potential, but, seriously, a NEW Gamecube is $99. It's a steal considering just the list of exclusive titles above, not to mention Zelda: Windwaker, Metroid Prime, Factor 5 Star Wars games, and all the usual suspects that appear on all three consoles (i.e. Madden, SSX, Tony Hawk, etc).
If $99 is a steal, it's certainly at least worth $60, which is the most you're going to get for a used one.