Tinkering inside a Mac didn't used to be as much fun as tinkering inside a PC was – that was part of the reason one would choose a PC back in the day. That's changed, and now grubbing around inside the Mac Mini can net you quite a bit, including “unofficial” upgrades. It's possible to convert
a 1.5GHz Mac Mini single-core system using the Intel CPU, to a dual-core 2.16GHz processor, definitely noticeable in the Mac world. The stock heat transfer system in the Mini is more than enough, apparently. Though this means you'd have to go out and buy another chip and other components aren't so easy to replace, one upgrade is better than none! The linked article has photos and a guide if you are interested.