Hey guys- thanks for the repies. An update: got a B & W mac running at *edit* 400 mhz off ebay. Don't know when it will be coming in, but I've already started to think about the parts to the cooler. The only thing I've decided on for sure is an Eheim 1048 pump.
As to whether it's worth it or not, I'm a tinkerer at heart and I enjoy doing this. It's also about gaining experience, since I've never set up a watercooling system before, and I might as well do it on an older peice of hardware.
And G3 chips are very overclockable in my experince. I'm typing this on a blueberry iMac that's been pushed from 400mhz to 500mhz with passive cooling alone. I'm pretty sure I could get something like a 50% overclock with watercooling.