I know for the intel based minis-you can up the RAM-change the Hard Drive and the Optical drive.
For the C2D systems RAM can be upped to 4 GB-and I think to 2 GB on the CD systems.
I have seen 320 GB HDs that will work in the system and supposedly there are now 500 GB HDs.
There have also been Processor Upgrades, though I do not know much about these. I have seen a 2.33 gHz C2D listed on ebay in the past.