Windows 7 - Anyone with experience building high end workstations?

I got a call today from a potential customer who wants a machine that can run Autodesk Inventor without bogging down, but he doesn't want to pay 20K+ for a workstation from HP or Dell.. After hearing what he's currently using for hardware (enough to make almost any gamer start drooling), I had to a little research. This is what I've come up with so far based on what he's asking for.. am I missing anything?

Intel Xeon X5680 x2
EVGA Classified SR-2 MOBO
Kingston Memory Model KVR1333D3D4R9SK3/12G x4 48GB
EVGA Classified SR-2 100-PS-1200-GR 1200W
PNY VCQFX5800-PCIE-PB Quadro FX 5800 4GB x2
OCZ RevoDrive OCZSSDPX-1RVD0240
Western Digital RE4-GP WD2002FYPS 2TB SATA 3.0Gb/s 3.5" x4 (for RAID 10)
LIAN LI PC-V2120B Black Aluminum ATX Full Case

Based upon the system requirements for that product, it seems that you might be going way overkill on this build
Autodesk - Autodesk Inventor Products - System Requirements

Even with complex modeling, it says 6GB of RAM or more. I don't think you really need 8x more.

Possibly. The machine the customer is using has 24GB of RAM and still lags. Even with 3GB of video memory and a core i7. Does anyone here who really knows hardware like this actually use the software I'm asking about? It's not a big deal except that I don't want to build something that isn't going to still be up to the task two years down the road.

I could look up all the bits, but just out of curiosity -

What does a rig like that cost? It indeed looks like a $20k machine, if purchased from HP.

I don't suppose that you'd try to install Windows 7 Home Premium to save $100, but you probably already know that you'd need Pro or Ultimate (or, I think, Enterprise) to support the two sockets.

I told the customer I could do it for $17500. HP wants $25000 for the same specs, if slightly different hardware...Of course I'm not going to skimp on the OS
It's just under $14000 for the parts, unless I've forgotten something. One of my concerns is will it be difficult to use the revodrive for the OS and programs? Otherwise I will order a pair of SATA SSDs instead. The four HDDs are intended to be used primarily for storage.