Why don't you buy something like a barebones Dell T5500 or better - T7500.
Curious to know why you need 4x PCIx16 slots? What could you possibly be putting into the 4th x16 slot that doesn't also come in an x8 or x1 interface?
What are you thinking for stands for this monster?
Ergotron has a few options that may actually work for you... (this is scary to think about on one ergo stand...lol but it might work)
Ergo makes different sets of brackets now - you have to call their customer support to order them but you can get them. They have a standard double and a standard tripple - but they have also started to produce "extra wide" double and triple arms.
They also make longer upright extensions (kindof custom to order if you talk nicely to the support people).
You **MIGHT** be able to purchase 3x extra wide double arms and 3x extra wide triple extensions and one extra-long upright pole to fit 4x3 monitors. (4 wide 3 high)
Or they make a 4-wide solution that's a little more stable - you could just patch that together so it has two bases (much safer IMO).
Regarding the PCI form factors - I think you are getting confused
There are three factors: Power, Form Factor (size of the slot) and Speed.
Look at this link:
http://t0.gstatic.com/images?q=tbn:ANd9GcQ2jZLXB7sgjnsxxGeNJNmT0vU8_nNZqenpviQX3vnfA_a2pw6NCA
that is the "form factor" or size of the slot. you need x16 FORM FACTOR because of the size of the slot & pins on your video card.
Your card draws a maximum power of <20W (call it 20W) so if you run three of these that's 60W total (needs to be factored into your PSU selection).
Look at this link for 20W reference:
http://www.amd.com/us/products/workstation/graphics/ati-firemv-2d/2460-mv/Pages/2460-firemv.aspx
Google is your friend on this by the way...
Industry standard is 25W on any x16 slot so when it says 4 pcie x16 slots but two at x8 that means you have 4x slots at a max of 25W (which meets your power requirements) but that only two can run at x16 speeds so if you use all four 2 will run at x8 speeds - but still produce the full 25W.
Read this:
http://en.wikipedia.org/wiki/PCI_Express
FYI - something like a Dell Precision T5500 or T7500 will do exactly what you need.
Also: you should also spend some time on Google looking up the ergonomics of bending your neck that high.
With those dell 24" wide-screen monitors you can't fit it all into the over-extended double+triple arms, you are going to need to get the 4-mon solution from Ergotron and then stack them 3-high.
What are you looking at to need that many montiors? I stopped at 6 and built another standalone machine with 4x monitors just because it got to be such a pain in the ass moving the mouse from one end to the other.
