Keep the power supply all you need is a couple 6 pin connectors per card , coolermaster is a great unit , you can always add more 6 pins if you need too .
I run 2 ati 5770 , 1gb ddr5 etc. etc.. so far have had no problems , crossfire set up fine and I have been able to overclock them when needed (such as when processing satalite images quickly ) came in handy during the christmas eve blizzard where in white out conditions rescue personal needed help locating stranded motorists --we were able to use thermal to locate, then pop in the gps location to get to them.(Wife processed the images on the rig I had just put together with the 2 5770's in it and sent me the data I needed to assist in the field to my truck computer)
I prefer ATI graphics, wife prefers nvidia the 250 is also a good card Just remember having SCooBY spam you with "ATI Owns you !!! " can be annoying