Finally what services are you going to provide. Next you need to decide on makes and models of hardware. This may be important if you are looking at doing lots of scientific applications. It will probably change again in a year or so. The choice of the right operating system is at least as important as the right hardware, and is actually rather intertwined with that decision.
Some operating systems work better with certain hardware and provide better services than others.
There is a drastic difference between an OS for a client machine and an OS on a server machine. For client machines, you're choice of an OS is often dependent on the users of those clients and the services and applications they need to use. For servers however, your choice should be one of stability and performance. If price is a concern, a BSD or Linux derivative may be a good solution. The Right Applications Applications can depend as much on OS as an OS depends on the hardware chosen.