This is very doable. I've got a Dell Precision 690 at work. It has 2x CPU sockets and 16x FBDIMM slots. The FBDIMMS slots are in riser boards that stand up off the motherboard. It's pretty similar to this Tyan board:
TYAN S5382WAG2NRF
I'm sure you could build one yourself, but I'd be a bit concerned...
- How do you support the riser cards in the case? They stand many inches off the motherboard & probably need some kind of a rack or support
- FBDIMMs use a lot of power. The Dell riser boards have their own power connection from the CPU. How do you make sure that you have enough air flow to cool fully populated riser boards in a home built case?
I think the easiest thing to do would be to buy a stripped down Dell Precision 690 (make sure you get it with riser cards!) and then add the ram later. The 690 starts at about $1900. Get the base model with the 1000W power supply and a low end cpu, video card, & HD. That'll be about $2000. Maybe you should just go ahead and get the CPUs at Dell if it's not a huge ripoff. Then add your own ram:
32 GB = 16x 2GB FBDIMM = 16 * about $100 = $1600 for 32 GB!!!
If you want to add your own 2nd CPU, you'll have to buy a spare heatsink from dell. I'm sure they'll sell you one -- you probably just got to call them. You have to use their special passive cooling tower since it's designed to work in the airflow of their case.
Go for it! I bet you can get a 8 core, 32 GB ram system for about $5k in a warranted & supported dell box.
-Knavish
BTW, what OS are you going to run??