hey folks:
Just my two cents here - Lots of very good advice for iamjero already, but two sites might help anyone else looking at this thread - for gaming PCs,
www.sharkyextreme.com puts out a bargain, midrange, and extreme gaming machine outline every month - they take a fixed budget ($1000 for bargain, $2500 for midrange, and $4000 for extreme) and pick components for both an AMD and Intel system at those price ranges, and explain the pricing and the reasoning behind their choices. I put together a system based on their bargain outline and have been very happy with it - runs every game I've put into it well, although for video editing it's probably pretty useless. Anyway, it's a place to look.
Also, living in Canada I find it very difficult to get good prices online, since the vast majority of sites are aimed at the American market. But if anyone is looking for good selection and prices in Canadian, I found
http://www.ncix.com very helpful.