http://www.pricewatch,com
I get a lot of stuff from there. Oh, and if you are going to get a new Motherboard, you might have to buy a new power supply and different memory...so it's up in the air. But I would sell the PC you have, and use that money to buy parts. Usually, people will pay more money just for the complete PC. Then, you put a couple hundred in from your pocket, and you'll have a kick ass computer.