You're in the position of a guy who owns a car with a normal V8 engine, asking about disabling half of the cylinders because some race cars use 4 cylinder engines.
Basically you can't get there from here, and frankly there is no need. You won't get more L2 cache per core by disabling half of your cpu any more than pulling out 4 spark plugs from a V8 engine will make the other 4 cylinders more efficient. Clock speed is essentially your only concern, and you can address that by smart overclocking or by buying a faster cpu and then overclocking that one.
The dual core cpus come from the factory clocked faster than the quad core cpus, and they will generally overclock to a higher clock speed as well. That said, you ought to be able to overclock a quad core cpu to at least as fast as the stock clock speed of a similar dual core cpu, with very little trouble. With a good heatsink/fan, a decent motherboard, and some smarts on how to work with memory ratios, you should be able to overclock your quad core cpu until it's faster than almost any retail dual core cpu's stock settings.
And you'll still have 4 cores instead of 2, which means some software and your operating system will probably run smoother/faster especially when you're running more than one program at once. Finally, if you're concerned with AH performance, AH uses all cpu cores so although you might get slightly faster AH performance with a really fast dual core cpu, you're going to be just fine with a quad core cpu. Overclocking your quad will just be icing on the cake.
I went with a dual core E8400 for price/performance, bang for the buck, nothing else. But if I really wanted more performance, I wouldn't consider for a second buying a faster dual core cpu. I'd buy a quad core cpu and overclock it.
In my opinion, the dual core cpus are better for "value" and price/performance ratio. Many of the people buying dual core cpus for raw performance are people who don't actually PLAY games, rather they just want to run benchmarks and brag about how fast their system is. If you want more cpu power and have the money to spend, you're best off getting a fast quad core cpu and overclocking the heck out of it. That's because more and more software and games are using all available cores. Don't get me wrong, you can build a very very fast system with a dual core cpu. But at some point you'll benefit from more cores or a more efficient architecture instead of just higher clock speeds.
If you really feel shortchanged in the cpu department, toss out what you have and get a top end i7 based setup. Then overclock THAT.
