[ddayson]

Hmmm, we're running three 3.4Ghz i7 Dual Threaded Quad Core (8 cores to PT HD), PC based PCIx HD4 systems with UAD-2 Octo cards, and they work great in our experience (PT HD 8.0.5/9.x).

Fast PC's with native PCI slots are harder to find, but still out there a while ago, and we used...

Motherboard:
JetWay JNAF93-Q77 LGA 1155 Intel Q77 - 4x native PCI slots, 3x PCIe slots

Processor:
Intel Core i7-3770 Ivy Bridge 3.4GHz (3.9GHz Turbo) LGA 1155

Memory:
G.SKILL Ripjaws X Series 16GB (2 x 8GB) 240-Pin DDR3 SDRAM DDR3 1600

Power supply:
CORSAIR CX series CX750 750W ATX12V / EPS12V

Case:
Rosewill Blackbone Black Steel / Plastic ATX Mid Tower (needs some internal trimming to fit all four PCI Cards, simple with Tin Snips).

OS:
Microsoft Windows 7 Professional SP1 64-bit - OEM

The builds cost about $1K each with HD's. These PT HD systems work great and have more power than we'll ever need, and we get around the 32 bit limitation with external PC VI Host(s). TDM has low latency for tracking, and the 8 Cores in addition to the HD/UAD-2 cards is more DSP than we'll ever use for music production.