From the list above I would go for Novatech, good prices and great service.
Depending on your budget I'd be looking for a Core 2 Duo with 2GB RAM, a couple of decent size hard disks, with one being used as a 'scratch' disk for video editing. An external hard disk for backup would be sensible.
The graphics card doesn't need to be anything special for video editing but I'd avoid on-board graphics.
My previous PC was from Novatech and I cannot fault their service.
My new PC was a custom build from Komplett and I haven't had any problems. After saying that I don't know what their service is like if things go wrong but they were recommended by a few people.