6600GT + new power supply + mother board = trouble

is there a way to know that the power supply is ACTUALLY pushing out 18 A at the +12V rail?

I have never tried to test the actual amperage of the rails on a PS, but I suppose you could either use a multi meter or perhaps one of the clamp on style amp meters, which would actually be the simpliest way, to test it. You would have to do this while the computer is under full load in order to get an accurate reading. If I am correct ( I am not saying that I am) you would choose the +12 wire of the main wiring harness that plugs into your mobo. You need to consult either your PS documentation or the manual for your mobo as to which one it is exactly. Like I said I have never actually done it before myself, but I would think that would be the way to do it.

What kind of PSU do you have? What are the advertised specs?

If you have a multimeter it's worth trying it. If you do not you are better off buying a new PSU because a NICE DMM will cost you more than a PSU would.

You can get a cheap DMM for less than $10 however do not expect it to be very accurate.

A nice true RMS DMM will cost you $50 - $150.

Semper Fi carry on