Yeah, I remember when I was using an HD 4850 by XFX for about a month, and I was running it on a 380W PSU without any lack-of-power issues.
Btw, a lot of people always say "Just go with a good brand" when it comes to buying a PSU, but that's just a rule of thumb if you don't know what you're looking for. If it's a card that needs only a moderate amount of power (like the 4850), I'm pretty sure you can buy a cheaper brand and save you're self some money. Just look for a PSU that's 500W and has 20A each on a dual +12V rail. Be sure to look at customer reviews, too. : D
Then again, if you find a really good sale on a brand name PSU, go for it.