To the best of my knowledge no gaming PC with anything short of a top-end or multi-gpu setup will ever even use 500W. But you want more than that to keep the PSU from running full tilt under load. Not only does it diminish it's lifetime, efficiency drops as well along with higher running temps.
I ran a 520W (Corsair built by Seasonic, HX520W) with an Q9550 @ 3.4GHz, 4GB DDR2, 8800GTS 512, 3-4 HDs, 5 fans, an add-in soundcard and over half a dozen USB devices. Used it like that 2-3 years without issue.
I have swapped it out for an Antec 650W now, but only so I could cobble together all the older components I had around into what now functions as a server 24/7. That old PSU is still going strong.
I've always used the Antec calculator:
http://www.extreme.outervision.com/PSUEngine
(please ignore the advertising, it wasn't there in the past
)
Just ran my setup through and it recommends 468W, 418W minimum (and that's assuming 90% of maximum theoretical use, which is not that likely to occur). Realistically, that would be enough, but it would still have the PSU at almost 90% capacity during peak load, so not ideal. That recommendation is really only the bare minimum you should look at.
The old 520W did warm up a little under load, but this 650W... it just doesn't bloody care. Could toss another GTX460 in there and maybe then it would start to heat up a little (ok, definetly would heat up a little, but still
).
Think a 6850 would serve you well, about the same as my 460 (little faster actually), no complaints in terms of performance. It's no monster, but
if I have to hold back it's never by much (1680x1050).