Once you know what application software and server OS you'll be running, you're ready to look at hardware. Most buyers select a server OS/hardware combination from a major vendor like Dell, HP, or IBM, or a package assembled by an IT consultant or reseller.
We highly recommend that you avail yourself of reseller expertise in translating your server needs into hardware specifications. Resellers can tailor systems to your exact circumstances and ensure that your server system grows with your business during the system's expected life. Even if you don't buy from a particular reseller, you can get good advice to take elsewhere. As a starting point, both your application software (a database or an e-mail server, for example) and your server OS will have recommended system requirements for a given number of users.
Specific considerations include number and type of processors (32- or 64-bit; Intel or AMD), amount of RAM, number of internal drive bays, and server design (tower or rack-mount). Intel's Xeon and AMD's Opteron processors are the standard for servers, and most servers come with at least two processors (each with dual or quad cores), which helps them handle multiple tasks and users efficiently. For storage servers, you'll want options such as hardware RAID support and external expandability, too.
At one end of the scale, a simple file-and-print server for 25 users should run just fine on an inexpensive system employing one dual-core Xeon and 1GB or 2GB of RAM. Since this setup would be used to store critical files, we would recommend using a mirrored RAID array to provide protection against drive failure. Moving up, an e-mail-and-collaboration server for up to 75 users, running Windows Small Business Server 2003 with Exchange and Sharepoint, might employ two dual-core Xeon processors with up to 4GB of RAM. And for a heavily used CRM database serving 250 clients, you might select two or four dual-core Xeons with up to 16GB of RAM, running the full version of Windows Server 2003. At the high end, dividing the workload across multiple servers--a technique called clustering--becomes more efficient. Load-balancing software is used to manage them.
If you will be using multiple servers, consider purchasing rack-mount models for ease of access and clustering applications. Tower models generally provide more drive bays and options, and they can be a more flexible choice if you have only a couple of servers.
Finally, don't neglect to investigate the warranty and support policies for your hardware and for the server OS. They will likely be of different lengths and from different vendors. For small businesses, which may rely on one or two servers for everything from handling e-mail to sharing documents and printers, downtime can be disastrous. Invest in same-day on-site service if possible, and guard against power outages by using an uninterruptible power supply.