I disagree. You could make a dual CPU Skulltrail clone!
It supports 2x LGA771 150W 400fsb CPUs. X5492
http://www.delidded.com/lga-771-775-cpus/?filter=socket-lga-771,socket-lga-775,series-core-2-extreme,series-core-2-quad,series-xeon,fsb-1333,fsb-1600,node-45nm,codename-harpertown#cpus
LGA771 CPU X5460 $14 9.5x1333fsb, or X5470 $35 10x1333fsb
http://www.delidded.com/lga-771-775-cpus/?filter=socket-lga-771,socket-lga-775,series-core-2-extreme,series-core-2-quad,series-xeon,fsb-1333,fsb-1600,node-45nm,codename-harpertown#cpus
The LGA771, and LGA775 CPUs are compatible in the BSEL, and VID pinouts. So a BSEL pinmod 333fsb to 400fsb will get you 3.8GHz, or 4GHZ respectively. To do this mark the locations of the LGA775 locating tabs onto the LGA771 CPU and follow the 775 pinmod maps.
VID pinmods are harder. But you can get control of voltage with Throttlestop software to support the faster fsb.
The wierd stuff= The 2nd CPU mounts on an add in board with added memory cooling etc.
The memory is FBDIMM DDR2. This is very strange stuff (Skulltrail used it). Each chip has it's own controller, and each channel sets up it's own bus. They run hot, use a lot of power, and have high latency. Fast throughput though.
It's available cheap. MacPro used it in a faster speed, also available cheap. Not sure if the MB controls the speed, or the controller on the modules. There's an experiment to be done there.
The PSU is going to be weird. The power hungry FBDIMMS, and the "daughter board" require proprietary connectors to run them.
Since these were sold with anything form 2 core, single GPU to 2x4 Core dual GPU setups, there were probably several PSUs available, and also several wiring harnesses to match. They will be seriously multi rail. you will need to be sure that if you try to adapt say 6pin GPU to 8pin GPU that there is enough power at that specific cable. But a full size powerful GPU should be possible.
Aside from the GPU $200 should do the rest.