I concur with Coloradojaguar....reliability in the parts is key. You would also want to make sure that the vendor you choose uses the same parts in the specific model you purchase. Do not be surprised if you find that with some vendors, slightly different part are used in the same make/model. This makes it more challanging for you when you are looking for replacement parts. I would stick with a vendor, that has an excellent reputation, and good service options for you. From my experience this vendor list is very short.
Easy access to the innards of the system, including disc drives.
Fully buffered ECC RAM - this is error correcting RAM which will continue to run even if one of the memory sticks fail, allowing you time to take down the system cleanly before you replace the part(s).
A big power supply (750-1000 watts).
I have an 8 core Intel server/workstation and these factors have saved my butt on more than one occasion. If you are going to run Linux, then make sure you install the lm_sensors module so you can actually monitor the CPU and RAM temperatures.