processor at good speed .. core 2 duo & above
sufficient ram .. 2 GB +
Graphics card ..
HDD with RAID(0 or 1) setup or more speed like 10000 rpm(like WD raptor)
64 bit operating system(if you are going for more than 4 GB ram)
a good video editing/encoding software with multithreading support
if you go for high end models the you will get more performance..
I use two video editing computers, both w/ 4 Gigs RAM - one a Q9400 OC'd to 3.2 Ghz and the other a Phenom II X 4 955BE OC'd to 3.6 Ghz. The Q9400 is faster vut only slightly.
I edit 1080p (Samsung UM10) and 720p (Flip Ultra HD) mp4 files and there's no way Core 2 Duo E8400 even OC'd to 3.6 Ghz could keep up.
In 3 months I'll probably go with either the new AMD 6core Thubans or Intel i7 860. If you're not editing or encoding HD, you're probably okay with 2 cores unless you'd rather wait 10 minutes instead of 20.