I dunno, the HP Pavilion isn't as bad as SOME off-the-shelf PCs, but it's certainly not very pretty:
Then you have the Lian-Li "Pretend G5":
This Cooler Master looks very good. Fit to carry a Mac, for sure:
My point is that this is your DREAM Mac, not your "it'll do" Mac. A friend of mine actually put his Quicksilver into a new case. But you can't cheap out when doing that - you have to get a very good case worthy of a Mac. With a PC, you can throw it into any old case. Get some off brand like "Powmax" or something, it really doesn't matter.