Would you buy the Mini to replace your MDD.
Isn't your MDD dual processor now? So your MDD with the L3 is going to be faster....
MDD has room for improvement in every sense where the Mini does not. 4 Hard drives vs 1. PCI slots vs no PCI slots. Upgradable video cards vs built in graphics/non upgradable. Lots of ports vs a few. 2 optical drives vs 1 slow slot loading drive. Dual processor option vs single. L3 vs no L3.
The mini is smaller, quieter, and more energy efficient that's about it. The PowerMac is a pro level computer the mini is entry level.
I think your better off with a smaller number of faster computers than a larger number of slower computers. You have a ~1.25 G4 PM DP, ~1.25 eMac, and now another ~1.25 G4 desktop. It's a lot of redundancy when you could try to sell 2 of those and get something faster. Just get a G5 already and call it a day. Just know it's not practical to get a 60 pound hunk of aluminum shipped from Ireland.
Don't you owe people money?