There isn't really any premade Video Editing Pc's I can think of unless you're willing to drop like 3 g's. The best thing to do would be to get a computer with a decent proccessing speed, a lot of ram, and a big hard drive. Ram and Hard drive size and speed are probably the most important things when it comes to video editing.
If you want it just for video editing then I would also say to go with a mac, but if you are looking for just a computer in general that also has good editing capabilities then stick with PC. Macs are good for productivity with editing and sorts but a lot of programs just aren't compatible with them.
Ya the more 'exotic' pc companies do have workstations for all that kind of stuff. I know of Alienware and Hypersonic who make special DV editing systems, Im sure there are more too, but they come at quite a hefty toll. Thats why I would advise just putting only the necessities for video editing in a cheaper PC that way you dont spend 4 g's.