I've made this statement here before...
I have an example of every G4-based Mac that Apple has made with the exception of the Xserve and the Mini. The Mini is a computer that's completely, 100% off my radar and I have no desire to own one.
On the other hand, I have a CoreDuo Mini running SL Server in the closet that runs 24/7 and has for several months now. I have zero complaints about it. SL is the most recent OS that will run on a CoreDuo processor.
BTW, quite a few C2D Macs are officially limited to Lion because they have 32 bit EFI(despite the 64 bit processors). My Mac Pro 1,1, late '06 iMac, and late '07 Blackbook are in that boat. There are work-arounds with most of these computers to install newer OSs, although the crummy GMA 950 GPU in the Macbooks, Minis, and some iMacs makes this less than desireable. The Mac Pro takes a minimal amount of hacking along with a GPU upgrade to install newer versions,
Interestingly enough(although not to get off track too badly) with most newer versions of OS X you don't even have to flash the GPU to get it functional provided that you select the correct GPU. The downside to not flashing is that you don't get a boot screen until the OS loads, but aside from that they work fine.