Again for any decent power supply it shouldn't be a problem but what power supply are you using, obviously a low quality 600w rosewill psu shouldn't be used to power anything more than a 5670.
Agree that their old units are bad, but the new ones like the Hive series are not that bad. Anyway for a PSU to handle 2 7950's i'd recommend a TX650/TX750 or better yet a HX650. If I was using a CX600 I would not put anything in more than a single 7870 because I on't trust the CX series. Also the OCZ ZT series are good and XFX. Enermax ans Seasonic also very reliable.