I use a EVGA 1000W which runs 4 x 280X cards running X11 (16.5Mhash) and 1 x Nvidia 750Ti card running X13 (2Mash)
I used to run the 4 x 280X cards on scrypt although it was probably at close to 100% load then.
Most likely that will be very close to load, if not over depending on what your doing.