Im in the process of selling my HD3 that ive used in a PCIX to PCIX magma chassis (P7R4) for the last year or 2 and worked great,
If you are going for PCIe-PCI-X you gotta get a magma PE6R4 chassis to be supported by digidesign,
There are some chapo chassis by a company called startech but they wont be qualified or suppoter by digi so they will point at that chassis if you ever have a problem and blame that,
THE PE6R4 have a 4x PCIe host card and the Startech only has a x1PCIe host card so you may get bandwidth issues,
I actually had an OSX Hack running on my PC with the PTHD for while which worked well (just dont like Mac OS so trashed it and kept WinXP).
Im selling my rig while its still worth money and waiting to see what digi come up with next too.
Go to the digidesign DUC forums there is plenty of chat about this sort of thing there by people with the setup you are discussing.