Search for and download the latest version of RMClock. It does function under Vista now and gives you voltage and clockspeed controls. I used it for a bit a couple days ago, but haven't really dug into it too deeply. Check it out! I don't have a link for it off hand but its out there guys.
NHC will have a Vista release very soon. The guys over there are working hard to make it compatible, but Vista is throwing many wrenches in the gears for them. The fellow that writes the software does it on his own free time, and usually without compensation so it will be done whenever it gets done lol.
Also, I heard rumors that the RivaTuner program may pick back up again soon... there are a lot of people pestering him about writing the new code, and of course there is no solid timeframe for a release. It just seems that he is taking a break from it for a while before stepping back up to the plate again. Keep your eyes peeled in the future for a RivaTuner comeback!