Why not get a current generation CPU instead? If you're going to be replacing your motherboard, why would you want to replace it with something that's only marginally less out of date? It's not like entry level SB chips are expensive. You can get a kick ass dual core for about $65, or AMD quads for around $100.
At any rate, it seems like you don't know much about computer hardware. I would very strongly recommend doing a bit of research before you go buying components you don't understand. Replacing a motherboard isn't a simple task for somebody who's new to this kind of thing. You'll need to totally dismantle the computer to do it, and if you're using an old case from Dell, HP or some place like that, you may not even be able to fit a new motherboard in there.