You donr "Have" to use an approved system, there are plenty of proven working self build configurations if you look in the windows HD section of the Avid/Digi DUC forum which would be cheaper and more powerful,
Or if you dont want to self build theres always this:
http://rainrecording.co.uk/software/avid/
Approved just means they have personally tested it in their workshop and the latest PT build has passed their tests ok.
Ive designed and built systems in a previous job as an R&D Engineer and had them approved by Digidesign, it doesnt mean an awful lot, except in marketing blurb.
You wont get any extra help from avid support by having an approved PC.
if you are capabale of building and configuring your own machine,
I would only go the HP or IBM route if you have no idea how to build and config a PC.