No sorry,if your system came with a single CPU,then thats all the Mainboard would be built for.To run a Duo you need a Duo capable 775 socket and the appropriate Chipset too.Also a normal Dual Core Mainboard cannot use the new Conroe Cpu,the Mainboard has to be Conroe Certified and with the correct Chipset too.A Conroe board can use both types of Dual CPUs.Same rules for AMD too.
