Instead of disassembling your computer, you could right click on "My Computer" and select Properties (or do Windows logo + Break), and then go to the General tab. At the bottom, it should list your processor speed and RAM size.
I concur with surgeVel on the sites, but I wanted to also list Pricewatch.com as another site to look for memory.
One thing to be aware of is what your motherboard can handle - you don't want to pay a lot of money to get 8GB of RAM only to find out your motherboard can only handle 2GB!