Your best bet is custom made, that way you can get what you want (graphics card, RAM + hd) . I've got a custom desktop for around 600, i5 quad core, 8gb ram, 3,3ghz 1tb hard drive 1gb nvidia graphics card. I'm a graphic designer so I need photoshop and illustrator etc running alot of the time, even 4gb quad core would be okay. But depending on what sort of editing + software your using if its heavy editing then I would say i7 quad core with 8gb + ram just to be safe.
If your from the Uk go for arbico.co.uk but I think your canadian? so maybe go for newegg.com or for specific parts try craigslist?
Peace-
Paddy
Full PC specs are on my channel; http://www.youtube.com/user/byZPGFX?feature=mhee