Well, due to my calculations i've even run a nonlagg noncrash server on a single 2ghz CPU with 256 mb ram, the server + 12 people only caused trinitycore.exe ( i used that ) to take in around 90mb and an average server load of 3-13 % ( the load increased when we mass teleported all together to highly mobbed areas xP )
Hope this answers your question.
CPU isn't that big, but as said bigger the better, also, each players consumes roughly around 12mb ram, this could increase to 20mb ram, although a friend of mine had 25mb ram per player, so i think it's what server you use.