The G5s do come with USB 2.0, but any powermac is probably gonna be a snap to pop a PCI card in to. If you need the fast USB, don't go for a notebook, iMac or eMac. Eh, maybe a PowerBook, if you can get a PC card for it.

For file sharing, I'm using "Poisoned," which can use several different networks, including FastTrack, which is what Kazaa uses. It's real nice. I've tried some file sharing things for Linux, but they were hard to set up and rarely got a successful download, so I wasn't looking forward to using one on a Mac, but it was easy to setup and has a great rate of successful downloads. It's about as good as Kazaa when downloading from a FastTrack user, but you also get the other networks, and they're often successful, too. Like some sorta Kazaa+. But not exactly a massive +, just a little one