GTX 295 will work on Corsair HX620W?

Nvidia say it needs a 680 Watt power supply or a supply giving 46 Amps yours gives 50 Amps so it should be ok.

Requirements
Minimum of a 680 power supply.
(Minimum recommended power supply with +12 Volt current rating of 46 Amp Amps.)
An available 6 pin PCI-E power connector and an available 8 pin PCI-E power connector

You have two 8/6 pin connectors as well so its good to go.
http://www.corsair.com/products/hx/default.aspx

But you knew most of that already ;)

Mactronix