OSX on atom
I've been running OSX on my eeePC 901 with a not so powerful 1.6Ghz N270 Atom chip, and it's fairly responsive, particuarly with 2Gb of RAM in there. Having graphics with Quartz Extreme support offloads most of the fancy eye candy from the CPU. I can watch avi's streaming over wifi with no problems.
I've also got a 2.0 Core2 duo mac mini, and that does me fine as well, so perhaps the Atom 330 (64bit, IIRC) will do a decent enough job, any maybe help to slim down the mini even further.