Maximum Graphics Card Power (W) = 170
Minimum System Power Requirement (W) = 500
So 500W for the single card set up and 670 for an SLI setup
If overclocking both GPU and CPU, I'd go 650 for single card and 850 for SLI
If ya haven't bought the case yet, I'd consider this combo ... case is outstandinghttp://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.1278880
$280 MSRP - $30 new egg discount - $20 combo discount - $30 MIR - $20 off w/ promo code EMCXSXL29, ends 4/15 = $180
The Corsair X series and Seasonic X series is a step up .... Comparable models to the TX series would be the Silverstone, Cougar, NZXT Hale, XFX Core Edition, Antec HCG series .... See jonnyguru.com for reviews.