What version of Debian are you running? XFree86 is obsoleted nowadays by X.org (though I'm pretty certain it didn't make it into Sarge).
I would recommend checking out Ubuntu. Should run fine (at least as well as Debian) on the iMac, uses the apt-get and whatnot, and is a breeze to use. 5.10 was just released a week ago or so, so it's very current.
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