If you move to a 4,1 or a 5,1 you'd be able use any disks, pci-e cards, etc that you have in the 1,1 now.
You would still have access to an extremely wide choice of graphics cards at a very wide choice of budgets - see what a fast graphics card will do to Lightroom or Aperture in a 1,1, it really is quite astonishing.
You would be able to have 4 hds, and 2 optical drives within the case - as now. Unlike the nMP which needs it daisy chaining outside the case.
Else if you want to do cheaper, the Mac Mini is due a refresh so that may be worth considering?
-Rob