AMD’s triple-core X3 CPUs are actually a quad core CPU with one of the cores disabled. According to Playwares, you can enable the fourth core of the Phenom II X3 710 simply by selecting Auto in the Advanced Clock Calibration setting on a Biostar TA790GX 128M motherboard.
The site has posted some interesting screenshots that show a test system’s start-up screen before and after the BIOS change. It starts off detecting a Phenom II X3 710 that’s been overclocked to 3.12GHz, but after the BIOS setting it then detects the CPU as a 3.12GHz Phenom II X4 10, complete with four cores.
The site has also run CPU-Z on the CPU before and after, which appears to show the same CPU with four cores after the BIOS change.
The site also ran the CPU tests from 3DMark06 on the system, and the score increased from 3795 to 4776 with the extra core enabled.
According to Bit Tech it is in theory possible that the 4th core with have stability issues. AMD use a native quad core system which means if one core if faulty they could just disable it and sell it.