Provided you don't waste power by e.g. by CPU mining Litecoins, easily.
A 5970 shouldn't pull more than 250 W with its memory underclocked.
With 120 W for the rest of the system, the PSU load would be 82,7% - not unreasonable for a high quality PSU such as the ZT 750W.

Make sure that the CPU power save options (such as SpeedStep or C1E) are enabled in the BIOS.