I would strongly suggest something over 500W.
The card requirements are for a 500W PSU, but you have to assume that the PSU is not necessarily running at max.
If you are getting a very high-quality PSU, such as a nice Corsair, 500W will do the trick.
Otherwise, Newegg has some pretty good deals on ThermalTake PSUs right now - I just picked up a 600W TR2 for about $40 after discount: http://www.newegg.com/Product/Product.aspx?Item=N82E16817153166&Tpk=thermaltake%20tr2%20600w
It runs my Sapphire 7870Ghz OC 2GB just fine.
Now if I can just resolve that driver issue...