As a tech I don't use any. At a cost of $500us+ it's cheaper to keep a handful of spare parts around (from other dead systems) than to have a diagnostic card around. But one that I've seen and tested is the Ultra-x P.H.D. PCI from www.ultra-x.com it's discontinued now but they have a newer version available. As far as diags, start witha minimum config, motherboard, video, processor, memory only and then see what happens.