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.
Okay, so I got my 2nd 5970 today, running both @ 820MHz Core/300MHz Memory with the CPU having power savings enabled. No problem with my OCZ ZT 750 Watt handling this, getting 1400mh/s.
