There is a thread where it was agreed upon that the older minis could not play Blu Ray rips.
Doesn't make sense, it depends on how you rip them of course.
1080p x.264 AC3 MKVs (ripped from blurays) that you see around the web usually work fine on my 2.0ghz 2009 nvidia Mini..... (edit: sorry, maybe with "older" you meant GMA950 Minis)
More over, Apple has just released an API for 3rd party programs to take advantage of h.264 hardware acceleration if you have: 9400M, 320M and GT 330M.
There's already a beta of Plex that use it, soon we will see it in a final release. (but you can already use the beta). People testing it say that CPU load is drammatically reduced and there are no slowdowns on the heavy videos.
So MacMini 9400M/320M + new Api + Plex + Remote Buddy + Wii Remote = damn good mediacenter IMHO.
Now with HDMI (video+audio) to run just one cable to the A/V Receiver. Not that running a Toslink cable is a problem for me. It's not like I'm moving my mini all the time.