Today Legion Hardware has a $280 US motherboard and processor combo for you that can be turned into the basis for a high-end gaming machine. Without having to upgrade the cooling we were easily able to push the Core 2 Duo E4400 processor from 2.0GHz to 3.0GHz which significantly boosted performance!
"The E4400 is a fantastic Core 2 Duo processor for overclocking as it is essentially an E6700 with just 2MB of L2 cache on an 800MHz FSB. Given both processors share the same clock multiplier, all it takes is for the user to adjust the FSB from 800MHz to 1066MHz and there you have it, a 2MB E6700 for $140 US. Not bad given you would pay much more than twice this dollar amount for a genuine 4MB E6700."