If I wanted to, I could make mine go faster as well, but I'm worried about the heat output.
The Core 2 Duos are the leaders currently, followed by the X2s. I'm fairly sure AMD will make a beautiful come back. And the new processors, if they're AM2+, they're backwards compatible with AM2. As for going quad core, make sure you have a use for it .
I had a very good reason to go for quad core...... Supreme Commander and UT3!