|FPS depend more on the video card than the CPU. Which video card do you have? Do you even have a "card"? If you're still using the onboard Radeon 3100, that's your problem.|
As for the number of cores, run MSCONFIG, click the BOOT.INI tab, then click Advanced Options...is the box next to /NUMPROC checked or unchecked? If it's checked & only shows one processor, uncheck it & reboot.
If you upgraded from a single core to a dual core, you may have to reinstall Windows.