If not OC'in then which CPU?

Nope. Get the Dual.

The E8400 would be a better gaming chip today, but in 2-3 years' time you would want a quad and I bet it would be very hard (and expensive) to find an LGA775 quad-core processor since LGA775 is going the way of the dodo in not too long. A good parallel to your situation would be somebody looking at getting an AMD socket 939 unit in late 2005 or early 2006, right before it was phased out. Today, it's very tough to find dual-core socket 939 chips and the ones you can find are usually at a ridiculous price. You can get decent quad-core CPUs today for well under $200 and they will give you almost all of the gaming performance an E8400 will today, and much more later when games use more than two threads. Just look at the "Can Your Athlon 64 Still Game?" article that was up on the front page a week ago. Back in 2005/2006, the 2.4 GHz/1 MB L2 4000+ single-core outperformed the 2.2 GHz/512 KB L2 X2 4200+ in most games but today the tables are much more than reversed in favor of the X2. Given the current trends of the increase in core count far outstripping the increases in clock speed, I doubt that tomorrow's games will be anything less than heavily multithreaded.

Since you are not overclocking, I suggest getting a Phenom X4 9950 and either a 790GX motherboard or a 780G unit rated for 140 watts. The X4 9950 is pretty close in performance to the C2Q Q6600 but generally costs a little less and AMD motherboards cost quite a bit less on average than Intel boards. Plop a new GPU in there every other generation and you should be good for 2-3 years and not drop too much money, which sounds like exactly what you're aiming at. If you have your heart set on the ASUS P5Q-E, then I'd suggest you get a Q6600 as the CPU. It's a decent CPU, has a good amount of L2 cache, and is one of the least expensive if not the least expensive quad-core CPU Intel makes.