Well the MDD was the last desktop Mac to support OS 9 out of the box, but if you're feeling bold, you can also push it into territory only a G5 was ever supposed to be able to achieve. It's all about what GPU you want for it, which can be far beyond what was ever officially supported, and how far you're willing to overclock it.
As others have said, the PSU is the weak spot, and I do consider myself to be lucky to have a model with a refurbished one. They even managed to tone down the "wind tunnel" on mine before I ever got it, though replacing the single 1.25Ghz with a double 1.25Ghz cpu card seems to be a little too much for what it's got, cooling wise.
I assume I can get a USB 2.0 card, Airport and better graphics, all PCI?
Oh, and for the record, you're going to be looking for AGP video cards, but otherwise correct. Don't worry, there are actually more AGP options than there are PCI ones. The only caveat there is that you're going to need a different monitor if you go down that rout, as the Apple Display Connector was only used on a handful of cards, and it's not really in line with the AGP spec. But, again, there are workarounds on using newer cards.
It's also far better set up as a file server than any G5, with 4 full sized hard drive bays, the only real limit there is that it all still uses IDE, and not even always the highest speeds possible. There are ways around that, but a SATA card would be optimal, if you can find or make one compatible.
The best news is that it has so many PCI slots that even if you replace all the older standards with new cards, you will struggle to run out of space inside of the thing.
I absolutely consider one a great project, and with the right upgrades it can easily keep up with a lower end G5. As long as you're willing to flash cards for the MDD, the cost on parts shouldn't get too high, PC parts from that era are still pretty cheap in comparison
