The multiplayer in Halo is fun, but the single player didn't strike me as being anything more than a really pretty FPS.
Glad to hear you're interested in a Gamecube! Hell, if I was you, I'd wait until August when Nintendo does their Metroid Prime bundle. It'll be $99, the actual casing will have an MP theme (the circular plate on top will probably have Samus on it or something), and it will come with Metroid Prime and a playable demo of Metroid Prime 2. If I didn't already own a GC, I'd be salivating at the thought of picking one of those badboys up.
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