I don't know if I am overstressing my PSU, but I have an FX-60 and a 2900XT on my 550W dual rail PSU. I has been perfectly stable in high demand tasks like Oblivion. But, better safe than sorry. If you can buy a new PSU, you should. And try to get one with at least one 8-pin PCI-E power (it only needs 2x6-pin, but you can use overdrive with 1x6 and 1x8-pin).