New EVGA gtx 260 - hard shutdowns

Hi,

I just bought a gtx260 to replace my old 8600gt and the problem started. I booted up fine into vista but when I start any game or gpu intensive program my pc will instantly shut down or shut down after a few seconds - 10 minutes, usually the latter. There is no bsod or error messages given.

I have a thermaltake tough power 650w psu, q6600 @ 3.2ghz, 4gb ddr2 ram and an asus p5n-t deluxe mother board. I've never had this problem with my 8600gt and the minimum wattage for the 260 appears to be 500 as stated on the box.

Everything I have is less than 3 months old..any help would be greatly appreciated.

There is also a faint to loud high pitched whining noise (like a kettle) coming from the psu....

EDIT: Okay this is my new theory, my powersupply supplies 18 A on both 12v rails, this equals a total current of 36A going to the card (the card requires 36A) Problem is this is pushing it and probably less then 36A. This would explain the instability when under stress, no? If so then i will need to replace my psu with one rated at 20A+ for my 12 volt rails correct? help is greatly appreciated and needed.