I'd like to setup a mining rig with 4 6970s (undervolted).

I have a 1000W Cooler Master Silent Pro (Bronze) sitting around - is this sufficient if the cards are undervolted?  Do I need to grab a better power supply?  If I do (I'm sure someone will recommend gold or plat) and I get something more efficient, is there a way to figure out how fast it'll pay for itself in energy savings?  (@ .12/kwh I believe)

Sorry if this is a double post - got an error message initially so I don't think the first went through.