From your location, I assume you live in Canada. Newegg won't ship outside the US. I've heard Tiger Direct mentioned for Canada, and there's probably others.
You'll probably want between 512MB and 1Gig total memory for your PC. Some PC's only come with 2 RAM slots on the motherboard, and there's a chance both will be filled. In that case you'll just have to replace one that's already there, so buy a little bit more.
If you click Start, Run, and type dxdiag in the box, and hit ok you'll bring up you system specs. It'll tell you how much RAM you have, but not what kind.
I've had good luck with Cosair, A-DATA, and Kingston brands.