850 watts and 1000 watts are way more than enough for any single current card. A good 650 watter is just fine.
With any current card but the 580, a quality 850 watter is plenty for SLI and overclocking.
For comparison purposes , the list below contains performance ratings (10 scale) at jonnyguru.com / prices as per newegg on 12/31/2010 and ones w/ double asterisks (**) made it onto silentpcreview.com's Editor's Choice List meaning they excelled at both electronic and acoustic performance:
Even the system they test on in the above article uses an overclocked i5. That being said a PSU is often more efficient when it's not being pushed close to full load so the OP may want to opt for a 1000W unit for this reason or if he wanted the extra headroom. If the OP is on a tighter budget however a solid 850W would meet his current needs and allow for an extra card in future.
Thanks for all the answers everyone. I had used 1 of the psu calculators but wasnt sure if it was accurate as it says 596w minimum. So just wanted to check. I will be going with teh 850 if i can find the corsair one i want locally. Thanks again for all the help