It depends on the game that you play,however currently most of the games benefit from a dual core CPU,but some games like GTA IV and some RTS games benefit from Multi-core CPUs so as some tasks like PhotoShop,3D Rendering,multitasking and etc so as i said it depends on what do you want to do.
However if i had to choose between them i would choose a Q8400 and OC it because you can OC a Quad core(Mostly) but you can't add cores to a Dual core CPU.
It seems with stock voltages you can get a Q8400 @ 3.0Ghz:
And to 3.3Ghz with 10% voltage increase:
It may not be as good as other CPUs but it isn't very bad

Here is another one:
"Unless you've been hiding under a rock, most people know that the Core 2 Duo 45nm dual core processors from Intel are high FSB screamers. Normally, this setup runs Quad core processors and overclocks them quite well. I knew slapping in the E8400 would bring out the higher clock speeds. I was not disappointed. After seeing the high VID this chip had I was a little skeptical about just how high I could get it to run. 500x8, no problem. It just took a little more CPU core voltage - 1.343v - than I had hoped. Things are looking up at this point. Getting above 500 was not too much of a big deal either. Just bump the vcore and move to the next level 5MHz at a time till it fails to POST. Above 500MHz I needed to adjust the MCH voltage, VFSB and the skew settings to get to 525MHz. Hoping to get higher, I was stymied by the the board or chip. Not sure which yet. To get that 525 x 8 level stable it took 1.45 volts to be Prime 95 stable. The maximum I could pull from this E8400 CPU on air cooling was a cool 4.5GHz. Unfortunately, it required 1.55 volts to the CPU core, but at this speed it was able to run Super Pi, which is good enough for the benchmarking crowd. Next stop, some P45 action. All in all, 525x8 or 4.2GHz, is a 1.2GHz overclock. 1.2GHz worth of performance increase ON AIR."