If it is the PSU problem, than i think you should try a Wattage calculator in internet.
All you can do is downclocking everything possible and try out how it will work, cuz overclocking sucks a bit of power from the PSU. Try it in bios for the CPU and with riva tuner for the card. Either way i don't really think it is the PSU problem. How did the x700 card perform before?
5670 card really doesn't take any real power from the PSU.
Downclock will definitively help you