I upgraded my Mini with a T7600 long ago, installed all ram memory it takes (2GB) and recently I installed a Seagate Momentus XT 500GB hard drive.
The Momentus XT made a big difference on the machine performance.
It was using Snow Leopard and it was fast, the only bottle neck is the graphic ship set, but there is no solution for that.
I manage to install Lion making some hacking to the installation app. It runs it fine.
It is a pretty good machine and it will hold for a bit longer.