There are server boards that run x16 on the PCI x16 slot, like this one:
I believe this is the same board on the 'Egg:
Also, do note that the 7950GX2 works a little different with the video RAM. It's not one card with 1GB, but two cards with 512MB each. Thus, this video RAM will be used in a different way than with a single card.
What is your idea about data security. I assume you don't want to have over a Terabyte of data on that thing and suddenly loose it all. How quickly you think you can fill up 2TB? I might recommend to get four 750GB drives and put them in RAID0+1 or RAID10. RAID5 could be an option also, but is not as easy to restore as RAID10 or RAID0+1 and generally has abysmal writing performance with onboard SATA controllers (although I don't know that last comment holds true for servers)