Purchasing a computer help...

[Mockery]

I'm gonna need to buy a new computer soon (well a separate one that I can do video/music editing on for my short films). Anyway, what I want to know is this:

Should I just buy a name brand computer? If yes, what brands are the best (and most affordable). CC sells Sony and HP, but I don't know much about those brands. If no, then where can I get a custom computer built for me with good parts (since I can't build one on my own without screwing something up I'm sure).

I'm trying not to spend too much cash on the thing, preferably within 800 bux. But I definitely need a good video card, a decent sized hard drive for those big video files, a speedy processor, and plenty o' ram.

Any help would be appreciated.