I sold my rev. D iMac a few months back and still have my iMaxpowr G3 upgrade card laying around. In the interests of full disclosure, I did have some occasional problems with locking up while the card was in the machine, but I was never able to determine if it was the card or a bad piece of memory I have since rid myself of. To ease the potential buyer's mind, I talked to NewerTech's new owner (Rick Estes -- great guy) and he said that he would repair any problems this card may have. My price is set low to compensate for any potential repairs. Again, you may not even need to send it in.
The card uses a 466 MHz IBM G3 and has a full 1 MB backside cache. My MacBench score nearly doubled with the new card thanks to the combination of speed and cache. The upgrade is fully OS X compatible and made the OS usable and UT playable. I also have a 128 MB stick of RAM I can throw in for a small premium. Email me or respond here if you are interested. My asking price is $115, as is, but I am willing to negotiate.