I have a 1350W PSU running 3x6990 just fine.  I don't have anything over- or under-clocked, and my hash rates are great (~2200 MH/s).  The system also includes an AMD Phenom 4C CPU at 2.66 GHz, 8 GB RAM, and other normal PC components to make it a usable system for gaming and such -- in case this whole bitcoin thing doesn't work out.