man this question gets asked like every other day now
it depends on what you're going to use the PC for.
PCSX2 emulator relies on faster clock speed, so the 8400 3Ghz would serve you better.
but for regular PC use and multi-tasking, the Quad-core would be better.
I would get the Q6600 with an aftermarket cooler and OC it to 3Ghz. (well actually i'd try and OC it close to 4Ghz, but thats cuz i like to push the hardware to its limits)