Not sure exactly how much detail you're looking for, so here are some general ideas and specs.
The only option available on the market now to built a 2spu 8 core workstation is the quad core Xeon Clovertown/Kentsfield processors. The lower end 1.6GHz 1066fsb E5310 sells for about $350USD with the higher end X5355 1333fsb 2.66GHz selling for about $1150USD.
You can get motherboards from Tyan, Supermicro, Asus, etc and they sell anywhere from $350 to $500USD.
If you go with the 3xxx series Xeons, you can get mobos that use standard ECC DDR2 667/533 memory. If you go with the 5xxx series Xeons you have to use FB-DIMMS. Memory can range in price anywhere from $50 for a no-name brand 1GB stick of ECC memory up to $150+ USD for a 1GB stick of FB-DIMMs. I recommend the 3xxx series because they are cheaper and use standard ECC memory, rather than the 5xxx series with the *generally* slower FB-DIMM memory.
A good first place to start is determining which series Xeon you want to use. Once you know whether your using the 3xxx or 5xxx series Xeons, you can determine what mobo you want to use. For mobos, you can check out Tyan, Supermicro, Arima, iWill, and maybe even Asus and Gigabyte. Personally, I recommend either Tyan or Supermicro. Once you know what mobo you want to use, you can determine what speed and size memory you want and as recomended by the mobo maker.
For a good 3D workstation video card, check out the nVidia Quadro series or the ATI FireGL series. The exact card and specs depend largely on what you are doing.
Another thing to think about is a PCIe or PCI-X RAID card solution. 3Ware and Areca are two brands well worth the money. Again, which RAID card and specs depend largely on what your doing. Same idea with what hard drives, capacity, and how many.
If you can provide some details and specifics about how the workstation will be used or specific software to be used, the forums folks would be able to provide some specific hardware recommendations.
Hope that points you in the right direction.