I would say that if you don't want to build it (which isn't as hard as some people think!), then get a Dell. Something about 40-80 Gb of hard drive space, 512-1000 megabytes of DDR RAM (Double data rate memory - very fast) and a 1.5 Ghz CPU, as well as a fairly good video card - a GeForce 3 at the least. Pretty much any operating system will work fine, but I would suggest Windows XP Home or Pro. They are generally more stable and are also faster than most others. Software wise, I would go with Ulead Video Studio 6 and Cool 3D if you are into things like cool titles and special effects. If you edit still images, I would say that Adobe Photoshop is a must.
Actually, for a video card, I would suggest a Radeon 8500 All in Wonder. It is a good card with all the extras that you would need to capture all your footage from your video camera. Also, make sure that the USB port on the motherboard is USB 2.0 not USB 1.1 It will make quite a difference on how fast you get pics off the digital camera.
Hope that helped!
Good luck - Jon