Seems to run everything fine with 512 here. UT 2003 is fine and runs exactly the same as on my XP2000 PC (mind you they have exactly the same graphics chip and the PC has twice the RAM), Myst IV is OK but has the occasional stutter (mind you I do usually run it on my G5 dual), iTunes ripping is at 8x (my iMac with Superdrive is about 12x and the PowerMac manages 27x so I put that down to the slot load Combo drive being slower which is typical of mobile components). iDvd seems smooth enough and I have yet to try out iMovie.
All in all I'd say the performance is on par with my G4 iMac although the GUI is slightly snappier even when running 1200x1024 resolution than my iMac at 1440x900, that must be due to the extra .4 Ghz.
As was expected the 80Gb HD is a 4200 model so disk access is not spectacular but that is something that I expect they'll fix pretty soon, 2.5" HDs are rapidly coming down in price as they are being used more and more in different products so the next generation Mini will probably have a 7200 drive.
I'm working up the courage to dismantle mine, wish me luck.
Amen-Moses