If you want Pro Tools on a desktop, best buy would be either a Digi001 or a HD system. The Mbox uses USB 1.1 (SLOW!! Unstable!!). The DIGI 002r and 002 uses both Firewire connection, this is better but not as good and fast as PCI. (HD and 001 is PCI)
Best buy would be a RME Multiface with PCI card! Buy Logic or Nuendo and you're ready to rock! But then you cant use Pro Tools. PT= fast and easy audio recording and editing, VERY stable Rewire engine. Downside, RTAS plugins use to much CPU. Search the DUC (
http://duc.digidesign.com/ ) for DAVEC test and watch the results and compare systems. Also watch the ""best desktop system"" (PC-section), page 1.