One of the best 24GB RAM setups you will find, is probably two sets of the Corsair Dominator GT 3 x 4GB RAM chips. Expensive though...
READ!!!, that RAM is and excellent RAM, but NOT for a server.
@OP: Assuming that you mobo only one CPU socket and four RAM slots, we need find the option to install the 24GB to run in dual channel in those four RAM slots. So, I need know the mobo that you have.