From talking about my hackintosh's so lovingly, there's one question I frequently receive from PowerMac G5 or Mac Pro owners: can I modify my existing case to use hackintosh hardware? Although I've long assumed that it wasn't worth the effort, it looks like I was wrong. If you're willing to make a few modifications to your Apple case, a true Hack Pro is well within your grasp.
A thread on the tonymacx86 forums points to some beautiful G5 case modifications and tutorials on how ambitious individuals have used Apple's cases to create some truly unique Hack Pros. If this is a project you're interested in undertaking, check it out and see what others have done. There are plenty of tutorials to get you started, complete with diagrams and instructions on how you can even use some of the parts already in your G5/Mac Pro (eg the fans). While there are certainly some nice cases out there, Apple's designed one of the best. You're in for a great hackintosh if you take the time to modify it.
The Ultimate G5 Mod Resource Thread [Tonymacx86]