seacliff :
HP ProLiant DL380 G7 with 2 6core CPU and a decent amount of ram will run any game server with a decent number of player on it.
That seems like over kill. I ran a 16man L4d Server fine on a Pent 4 machine....
Depends on what kind of server, how large of a server, and how many servers...
Remember that the following are not very important.
Graphics card
HD size (a 500gig is great.. anything over (depending on what your doing) could be a waste)
What is important
CPU (look a l1-l3 sizes)
RAM (look at front bus)
Ethernet Card (small servers you a 10/100 will work, but if you got $$ to upgrade to a gigabit switch a 10/100/1000 card is much better)
Good Antivirus protection - You may have to set your Server up outside your router's firewall for some servers to function.