If one is thinking of building, or buying a PC for video editing, Harm Millaard has published an ARTICLE in the Adobe Hardware Forum. He has arranged it such that one can rather pick and choose from three levels of system - beginner, intermediate and full-blown advanced pro-level rig. There have been some advances in hardware, since the article was first published, but with the discussions that follow it, much has been updated.
Harm approaches the computer from the eyes of one using PrPro, but with the exception of the GPU requirements for the Mercury Playback Engine (MPE), PrE will benefit from his suggestions. Be sure to follow all of the links that he offers in that rather longish thread.
If one is considering a RAID setup, his RAID ARTICLE will prove very useful.
If one is considering working with external HDD's, then this ARTICLE should be useful.
Hope that this helps people, who are considering a new computer for video editing.