2 cpu chips on 1 motherboard...

Not only do you need to buy a motherboard that has two CPU sockets (obviously), but you also need to buy CPUs which can work with multiple chips in the system. For example, the Core i7 chips cannot be used with more than one chip per system, but the Xeon X55x0 series (very similar to Core i7) can have two chips per system.

A while ago there was the Intel Skulltrail (an Intel board which could support two high end processors) which was cool and all but was limited by FB-DIMMs which were very expensive, weren't widely available and in some cases were hard to "tune" (in regards to timings).

A setup of two nehalem cpu on the same board (a server board would cost upwards of 900 dollars
The cheapest setup i can find is:

SUPERMICRO MBD-X8DTL-3-O Dual LGA 1366 Intel 5500 ATX Dual Quad-Core Intel Xeon Processor 5500 sequence (Nehalem-EP processor) ... - Retail
http://www.newegg.com/Product/Prod [...] 6813182187
340

Intel Xeon E5502 Nehalem 1.86GHz LGA 1366 80W Dual-Core Server Processor Model BX80602E5502 - Retail
http://www.newegg.com/Product/Prod [...] 6819117188
200 x2

+ RAM

For a higher clocked server nehalem you'd have to pay mulltiple times that amount
EG.
Intel Xeon X5550 Nehalem 2.66GHz 4 x 256KB L2 Cache 8MB L3 Cache LGA 1366 95W Quad-Core Server Processor - Retail
http://www.newegg.com/Product/Prod [...] 6819117182
1000 x2


Compared to 400-500 that you have to spend on the equivalent desktop ccomponents it is not worth it unless every bit of performance counts and you really need more than 8 threads.

Multisocket boards are typically designed for servers, and then, such boards tend to have an awful integrated graphics processor with no graphics slot, preventing their use as a desktop. Though, you may have a look, now that Pci-E is "often" capable of both - disk raid host and graphics card. But be careful, most chipsets can do both, but Bios not necessarily...

On the other hand, you get a lot more processing power from the Gpu than from any number of Cpu sockets. Some applications begin to take advantage of it, especially since DirectX10 hardware and Cuda libraries.

So maybe you could find such appropriate software? Then it runs on a graphics card (either nVidia or Ati, generally not both) or on a special GPGPU card, which contains one or more Gpu and no video output, like Tesla. Ati has similar offers, though they're late.