I used to use Riva on my old setup that had XP.
Vista x64 doesn't (or didn't at the time) allow the program to autoload at startup and that got a bit annoying.
I use Ntune now. Alot of people dislike it due to not being able to set fan profiles like you can in Riva. I've got the S1 cooler and turbo module on the card now, and it takes about 30 seconds to overclock in Ntune - which is simple and easy.
Load temps get to about 38'C during 2 hours of COD4 play - extremely efficient cooler, but then it should be for the size of it. I was loading out at 93'C with the stock cooler, not good.