It depends on what you have currently invested in your machine. Like MasterBlaster, if I had so much already installed in my machine, I'd rather get a CPU upgrade over a newer machine as well if the price was right.
By the way, MasterBlaster, the G3/800 ZIF from PowerLogix for just $410.00 CDN is a good deal, as well. As the G4 isn't much (if at all) faster than the G3 unless you use Velocity Engine-based applications. It's 512k L2 cache also runs @ 800 MHz. I was looking at this myself for the longest time.
Unfortunately, you're throttled by the bus speed - only 66 Mhz on a beige G3, 100 Mhz on a B&W. A 1 Ghz upgrade will certainly improve things, but it won't give you the same performance as a new 1 Ghz G4. I'd prefer to spend my money on an actual G4 - a Sawtooth is under $1000 around here these days. Then, later on, spend your next whack of cash on a 1 GHz upgrade for the Sawtooth - upgrades will be cheaper by then, anyway, right? (I know that the Sawtooth still has a 100 Mhz bus, but the whole motherboard architecture is superior, and don't forget the AGP!)
Normally I'd consider this upgrade but, after careful thought and research, it's really a bad idea. The bus speed is a big bottleneck but if you check out www.xlr8yourmac you'll notice some have already reported problems of a different sort -- heat. Basically the machine was never designed to deal with that kind of heat and, in the long run, you'll probably end up spending close to $1000 to fry your Mac.
If you have $800 to spend on Sonnet upgrade, then you have some money to call up Macdoc and see what he recommends. He suggested to me a wise upgrade to a blue and, if I wasn't saving up for a 1.25 G4, I'd do it.