I'll 100% agree with everyone else that if this is a server you are relying on, you really should go with an OEM vendor.
There are a lot of things different when you buy an OEM server vs. building one. For one, every part is custom and fits exactly where it is supposed to, cables are particular lengths, airflow is designed around the whole system, etc. Also parts are often very easy to quickly change out, and if certain parts fail, the server will keep running. (i.e. I have a server that can lose a CPU, a voltage regulator, a power supply, a hard drive or two, a stick or two of RAM, multiple fans, and it'll keep running.)
The only thing that I would add to the conversation is don't limit yourself just to Dell as the OEM. Personally I use all Compaq servers, and I couldn't be happier with stability and support. I get parts (when necessary, which is rare) overnight, and then I just peel the label off to reveal the ship back label and drop it in the mail, very painless.
IBM also makes some very nice servers, so just look around before deciding what you want.
If you decide on buying an OEM server vs. building one, check out the CDW Outlet (cdw.com, left hand side) for backstock items. These are usually open boxes, sometimes with a xeroxed manual instead of a regular one, but always with all of their parts and a full manufactuers warranty. The great part is that they often sell the items for less than cost to get them out of inventory. If you find something in there you want, PM me and I'll get my rep to hook you up.
Lots of things to think about, but I always recommend OEM to people that need to depend on a server, they usually turn out to be very little more money, and a lot more dependable.