The program it accesses for the orders, is it SQL based or what? SQL licensing is done by the physical processor, so two processors wil double the price of SQL. If this software was designed custom then I would contact the developer to see how processor intensive it is, and what they recommend. A server's performance is much more then just it's processor. You may wish to purchase faster hard drives, more memory, backup sytems etc. rather then just the fastest processors, especially if processor speed is not going to be an issue.
Hope this is helpful!
