How much is too much?

Right, first time building my own comp and I worked out (using the handy calculator link on this forum) that if I get a crossfire system setup in the future I'll need ~500W.

I understand that using less than total capacity on a PSU is good. So, if that's true and I'm not gonna draw loads of power or run crazy hot, then how much extra capacity is too much? I was thinking of getting a 635W...

Well, I'm looking at several models all between 550 and 700W. The 12V rails on all them are like: 12V1@19A or 20A, 12V2@19A or 20A, and -12V@0.5A or 1A.

Specifically, I was leaning toward the COOLMAX CXI-600B ATX12V 600W Power Supply 115/230 V - Retail (http://www.newegg.com/Product/Product.aspx?Item=N82E16817159055)



I'd be running a Core 2 Duo E6750, 2GB DDR2, (maybe eventually two, now one) Radeon X1950 pro cards, a motherboard to match, 7200rpm SATA Drive...

Thanks... may be a bit too expensive for my taste though. So if I ran a 750W PSU on my computer, which probably needs only 500W, I'd be fine? Does this cause some sort of inefficiency or heat problem?