How much wattage do I need?

its more than enough.

750watt would be more than enough to run 2x HD6850 in crossfire! This is about the most correct calculator http://extreme.outervision.com/psucalculatorlite.jsp

you can get away with a 450 watt one for that setup, (400 if its good quility) so thats more than enough.

Okay, thanks for the fast replies!

For a system using a single Radeon HD 6850 graphics card AMD specifies a minimum of a 500 Watt or greater power supply. The power supply should also have a combined +12 Volt continuous current rating of 24 Amps or greater and have at least one 6-pin PCI Express supplementary power connector.

The Corsair Enthusiast Series TX750 V2 (CMPSU-750TXV2), that you've selected, with its combined +12 Volt continuous current rating of 62 Amps and with four (6+2)-pin PCI Express supplementary power connectors, is way more than enough.