There are so many things going on with these boxes these days (memory mapping video RAM and such, I think), I have no idea why 6 GB is OK by 8 GB isn't on yours. Earlier versions of the Mini had that issue as well - http://discussions.a...=2571098&tstart=0 .
The Mini only has 2 sockets, so that's it (even if it took 8 GB modules that's way too expensive).
Older memory is often more expensive than newer stuff. In my quick browsing, I didn't see anything cheaper than what you found for your iMac. While looking around I found a thread about lots of users having freezing issues. I hope that doesn't bite you - apparently it was a video driver issue. http://discussions.a...tart=300&tstart=0
Be careful! Good luck.
Cheers,
Scott.