850w: http://www.newegg.com/Product/Product.aspx?Item=N82E16817152043
1000w: http://www.newegg.com/Product/Product.aspx?Item=N82E16817152044
Opinions please.
the 1000w got a "pass" from HarcOCP but wasn't impressive - read at least the last page: http://www.hardocp.com/article/2011/02/23/raidmax_rx1000ae_1000w_power_supply_review/9
For a mining rig it doesn't really matter. You're going to be running the PSU and GPUs 100% (probably OC'd) 24/7. Your main concern is a power supply that has sufficient amperage, and is rated to put out continuous power at high temps in the range that you need it. The cards are going to be getting f'd up anyway, a small amount of ripple really makes little difference to these set ups.
I mean, obviously the best quality is better, but a pass means that the PSU is solid enough to do its job, and that's what you need for a mining rig.