Gigabit Ethernet: If your network infrastructure supports Gigabit Ethernet, make sure your server also comes with Gigabit Ethernet capability to prevent your server's network connection from being a bottleneck.
Multiple CPUs: It's cheaper to put an additional CPU into an existing server than to buy another entire server, so multiple-CPU options for Unix/Linux, Windows, and Macintosh servers are becoming common.
RAID: If your server will be using internal storage, as opposed to accessing a NAS (see glossary), make sure it supports RAID (see glossary). One form of RAID known as RAID 5 is often used to boost both performance and fault-tolerance. Hot-swappable drives in your RAID can allow you to replace failed drives without shutting down the server.
RAM: One of the most critical aspects of sizing and configuring a server is making sure you've got enough RAM. One gigabyte is a minimum for any heavily used server, and 2GB or 4G are better. The good news is that RAM is still relatively inexpensive.