Re: I thought I'd seen it all...
Strange, my Mac Pro under my desk has had new drives (3 off, latest an SSD), memory, 2nd Firewire card added, UAD2 PCIe card, new video card (modded PCIe PC graphics card to get around Apple's ludicrous overpricing of ancient GPUs) added to it over the past 4 years.
I've just replaced it with a 4770K based hackintosh that pummels the base level new Mac Pro for performance, RAM and disk storage for almost exactly half the entry level price in Australia ($2000). This has a GTX 660 currently but wonder of wonders I can upgrade the GPU, RAM and add other drives trivially without having to add an expansion chassis. It's as quiet as my old Mac Pro and has been flawless since I installed it - which took 30 minutes, pain free.
And I can use it as my primary DAW system for the next 2-3 years and just upgrade parts as I need, even to the level of new mainboard/cpu. It even has a better warranty than Apple have just got busted for not supporting in Australia.
Also, Mac Pros aren't workstations, they don't have workstation level support from Apple outside the major centres in the USA. HP and Dell make real workstations with real certification and support.