First choose wich graphic card and editing software you need and follow the brand/model recommended on the corresponding websites. Try to have a bundle hardware/graphic-card/software from one company, as the setup can be really tricky (just forget about building it yourself).
Even if HP or Dell is not officially proposing it.,your local official reseller may be pleased to offer this kind of bundle.
We run Edit6 on a SGI-Pc with a MatroxDigisuite and a set of HD in Raid, delivered by a company, certified by Discreet, covering the warranty for the all lot.
Good luck, Guyb.