Avid usually codes their software to run on specific hardware. DS, for example, only runs on turnkey systems such as the Intergraph StudioZ and Zx10 as well as the new IBM stations. You may have to do some research into your desired applications.
In any event, if you're gonna do serious editting, get Ultra SCSI drives as they're much faster, have more bandwidth, and are (most importantly) more reliable than IDE based drives. Almost all serious turnkey systems use SCSI drives.
The key to good video editting performance is bandwidth. You may want to look at a 2nd hand SGI Octane if you go UNIX based. If you want PC architecture, then Intergraph Zx10 or the new IBM intellistations as they have the best internal bandwidth running over the 2GB barrier. SGI is up over 3GB.
When searching for a graphics card, make sure you get something that specifically accelerates 2D. FireGL and Wildcat are good in this department.
My 2 cents.