What PC are you starting with?  Or are you building from scratch?  The first thing I would do is measure the motherboard tray.  Go to http://www.newegg.com and check out some of the motherboards and their sizes.  This case is a tough build, as I put a G3 b/w in one.  Once you have a board picked out, print out a full scale photo of it and then you can start checking clearances.  PC are not Macs, but until I can run OSX on a PC build (yes I know its possible, but illegal-cough, cough and really doesnt run everything I need), it may be a while before I buy a new Mac.  It is so much more fun to pick the parts and build it yourself.

Post more if you need a hand in your mod.