Get an aftermarket cooler. It doesn't have to be a crazy cooler, just something that will do the job. The C2D's run pretty cool themself, they just need something better than stock. The Arctic Cooler Freezer 7 Pro is one of the best values for the core2. 3203 people can't be wrong. I had one and it was a GREAT cooler. And Flyin15 is right. a 3GHz CPU will run a GPU much better. not that it wont work now.
I have the e5200 on an Asus 750i motherboard. Just chaged two settings in BIOS and it took me 15 seconds to overclock it to 3.3 GHz at 12.5 multi X 266 fsb or 1066 quad pumped. I left everything else in Auto and the motherboard runs the e5200 at 1.2 vcore which is amazing for this kind of overclock! And the temps stay the same as on stock speed too which is nice work by the Artic Pro Freezer 7.
Wrong way around, you overclock, then find the lowest stable voltage.
There is no right or wrong way. Some raise to the max voltage, then up their bus speed and/or multiplier. Others, like me just start at the lowest voltages, and work the voltage up as you increase bus speed and/or multiplier. Doesn't matter how you overclock.
The end result is you stress tested the system and it is working without any problems.