Hello folks!
I'm an avid reader of this forum, but this is my first post here.
I have a few PPC Macs: Clamshell G3, PowerBook G4, PowerMac MDD G4 dual, PowerMac G5 dual. By the way, I'm writing this post from my G5

My G5 came with an ATI Radeon 9650. A few months ago, I was lucky enough to get an X800XT Mac edition in its original box; and just yesterday, a Geforce 6800GT just the card but in brand new condition.
I didn't have the chance to test all the other graphic cards. I've been focusing on this Geforce 6800GT because it is shiny, and smells like only new things smell... However, it was kind of a bummer when I installed it in my G5. It turns out I purchased it as 6800 Ultra; the antistatic bag was the original one I believe, but it had two stickers; one indicated it was a 6800 GT, while the other said 6800 Ultra. In any case, I got it at a very good price and in this condition... I mean, I cannot complain.
Thanks to this forum, I found info about
Graphiccelerator and
OpenMark. So, I decided to overclock it to at least reach the levels of performance of the Ultra. Looking at pictures of both Ultra and GT, I didn't see any physical differences. So, I'm assuming they're the same card but with a different BIOS, so flashing my GT didn't seem like a crazy idea.
Anyway, all this long intro just to share my results
PowerMac 7,3 G5 dual 2Ghz, 4GB of RAM, running Leopard 10.5.8. Screen at 1920x1080x32.
6800GT stock 350Mhz Core - 500Mhz Memory: Not tested (sorry)
6800GT @ 400Mhz Core - 600Mhz Memory: 13,436 <<< Same result as the Ultra, apparently
6800GT @ 410Mhz Core - 600Mhz Memory: 13,686
6800GT @ 420Mhz Core - 650Mhz Memory: 14,193 <<< Saw glitches running Halo
6800GT @ 425Mhz Core - 600Mhz Memory: 14,193 <<< Current configuration, stable on UT2004, Halo... no glitches
It seems the memory frequency do not affect much these OpenMark tests. Also, based on this experience, it is not a good idea to overclock the memory too high or glitches/artifacts start to appear.
Have fun!