Saturday for grins and giggles I powered up my
IBM eServer x series 330 1RU server (dual PIII-S 1.4GHz, 4GB RAM, Debian Linux) and ran it for a bit. Need a rare PCI-X GPU, though, for best performance. Fastest 32-bit PCI GPU is pretty slow for workstation purposes.
But I do servers for a living, and I'm as comfortable using that Supermicro X9QR7-TF as I would be using any other PC motherboard, even the old Biostars, Gigabytes, and others. I really like some of the server board features, though.
[Multisocket workstations] exist, but its a very niche market.
Perhaps I've seen some niche stuff, but I've seen quite a few Precision workstations, along with some
Intel S5000X dual Socket Xeon boards (a dozen of those at current $dayjob in the Intel chassis built for that series board). These can handle PCIe GPUs with no problem; for workstations you'll likely get a Quadro or a FirePro instead of a GeForce or Radeon.
At $dayjob my one Windows XP 64-bit experience was with a Dell Precision 690 with high-end dual Xeons, 4x 300GB 15K RPM SAS drives in RAID 5, a big ATI FirePro GPU with dual 24-inch monitors, and 32GB of RAM, ECC FBDIMMS of course. Used for astronomical photographic plate scanning and data reduction. >$10,000 workstation, grant-funded.