Apparently, changing the Advanced Clock Calibration (ACC) BIOS setting to "Auto" is all you have to do. It works on the Biostar TA790GX, Asrock AOD 790GX/128MB, and possible other 790GX + SB700 motherboards as well.
The site ran several benchmarks and it seems to work fine, the fourth core had no trouble with Prime95. A round of 3DMark06 benching further confirms the fourth core works properly, as it boosts the CPU score from 3795 to 4776.