To be honest, there is not a lot of difference between building a server and a desktop. The motherboards have more CPU sockets sometimes, the BIOS can be a lot more complicated on a true server board, and sometimes they use odd formfactors. But if you have good expierence in building desktops, servers should be no problem for you. The major difference between a server and a desktop is function and OS.
Just like in a desktop, there may be vastly different hardware specs on different servers depending on what they are doing. For example, at home, I have a Win2k3 server that handles my small home network's DHCP, DNS, file serving, print serving, and roaming profiles for two of my users (kids are always breaking things, at least roaming keeps their important stuff safe.)
All that for my small network at home runs on a P4 2.4 with 512 MB of RAM in an old Dell desktop I found. I just loaded the proper OS and whatnot.
At work, I have 20 servers that handle just my little OU on our big Active Directory stuff, each one is specialized because I have about 2000 users so I have more than one server for things like DNS, File serving, specific apps, e-mail and so on so forth. Each of these servers has at least 4 GB of RAM, at least 2 multi-core CPUs and most are in a cluster for failover protection and all are backed up both to a SAN and tape every day.
Both my home computer I described and each of those at work I described are servers. The difference in hardware is one of scope, just like your grandma can get by on a Pentium III computer for checking her e-mail whereas you need much more horsepower to play games and edit videos.
Just like in desktops, you have to make sure that all your parts are compatible. Make sure that your CPU/Motherboard/PSU/memory all work together. Do the homework and find the compatiblity charts for these parts and cross-reference them.
All of this is very, very general information. I am sure you can find many books about it. Toms Hardware had a great two (maybe three) part article on building servers that is a great starting point.