Kerry: I suspect the G3/350 only had a 512k level 2 cache, which makes a big difference. Come to think of it, the new 733MHz G4 processors have both an on-die 256k level 2 cache and a 1MB level 3 cache. With that in consideration the G4/733 would probably be pretty close to the performance of two 500MHz G3's for ProTools. Unfortunately the G4/733 processor won't be available as an upgrade for probably 6 months or so, and even then it will cost almost as much as it would to upgrade to a used G4/733 tower.
Mullet: Keep an eye on usenet, lots of people only advertise locally when they want to unload a Mac cheap. I'm in Canada so I can't offer much advice on which US areas are good to monitor. Keep in mind 500MHz G3 upgrades are only $259 at OWC (macsales.com), and zif carrier cards have helped maintain decent resale value on slower G3 processors. Be sure to get a rev 2 B&W, the rev 1's have a bug in the ATA33 controller. Check
www.xlr8yourmac.com to learn how to distinguish the two.
I hope that Digidesign gives us some warning when they eventually carbonize ProTools, so we know whether to hang on to G4's etc.