Agreed 100% here. The 68K machines are iconic and truly unique. Anything requiring OS X is compromised IMHO unless you can run modern stuff on it, too. In the middle, there's the Amelio-era beige machines running IBM PowerPC chips like that big beige 9600 box Travis is talking about. While interesting, I don't think they've aged well at all, certainly not for games.
when I had my 9600 I had os 9.22 on (with an installer patch, stupid apple) osx and debian lenny, and I rigged up a ATI radieon 7000 with a replacement bios to replace the horrid stock video ... it ran quake 3 quite well ... but so did just about every computer I had in the house (it was a little too choppy on the apple IIc)
but it was too new to run a lot of the classics, to slow to run osx and while it ran linux not very well