I tried it out on a Radeon 9700 and it runs great at 800 or 640. But I've been keeping it at 640 since it looks good and holds a solid framerate.
Today I stuck a Geforce FX 5950 Ultra that we had at work in my case to see how it will do at higher settings this weekend. It's pretty funny because I have a Shuttle case, and they're pretty small to begin with. I couldn't put the cover back on the case because the card is big enough to get in the way. I'd guess there's a pound and a half of copper on it.