Geeze, so much BS in one single thread.
First, an underrated PSU can NOT fry your system. Just would lead to instabilities due to insufficent power. But no damage AT ALL.
Second, combining 2 ore more PSU can give some very bad differences in electrical potentials if you don't make a proper low ohm connection of their ground potentials.
Third, the recommended specs for video cards are mostly overdone, so that even the worst low cost psu with the given (paper) rating could handle them.
Forth, the recommended PSU brands (OCZ, Corsair and Cooler Master) don't even build psus
at all, just rebranding them.
Corsair for example are just OEM seasonic (which itself is top notch, but the oem versions may vary) or channel well.
http://whirlpool.net.au/wiki/psu_manufacturers
Fifth, even the idea that a GPU can do ANYTHING to performance is.. borderline to say it the least offending way.