Well, after much dithering...mostly resolved when I finished filing our 2002 tax returns and discovered we'd be getting a nice refund - I took the plunge and bought a Sonnet Crescendo 1GHz upgrade for the old Sawtooth 400. Considered the GigaDesigns 1.2GHz card, but there have been wake from sleep issues with it, so went with the tried and true and slightly more pricey-per-MHz Sonnet. (Besides...I could drive up to Data Memory and get it - no waiting for UPS once the decision was made!)
Installed it and the WD 120GB drive I've had for months and noticed a fairly substantial increase in responsiveness under 10.1.5 in the 5 minutes I spent poking around before sticking the Jag CD in the drive and getting on with installing Jag.
WHOA! I have a whole new machine! (and I haven't even upgraded the stock video card either).
I think it's safe to say that it's now faster than my Ti, which I'd begun to use for all my web browsing because the Sawtooth was painfully slow. Not any more!!! Even Omniweb, which had never been exactly lickety-split in my experience, is pretty darn fast.
So, if anyone out there is thinking of upgrading a Sawtooth or something of similar vintage, I can't recommend it enough. For just over $500, I have my nice, quiet G4 with almost as much vrooom under the hood as the newer machines. I realize that some of the speed increase is no doubt due to Jaguar - but it was pretty clear even in 10.1.5 that this was a worthwhile investment. This should tide me over very nicely until such time as the PPC970 Macs hit the streets!