The third thing to consider is budget. It costs money to be on the cutting edge and probably isn't worth it anyhow. Another thing to take in to account would be future growth. If you anticipate hardware needs three to five years down the road you will be in a better position to maximize your return on investment. .
Now whether you build a server from the ground up or purchase one from a vendor such as Dell, Sun, or IBM you should know what hardware will be used in the system. The foundation can make or break a server. Without a good foundation your server could be prone to all kinds of problems. The first part of the foundation to look at is the case. The case will house all your components so you will need to make sure it has enough room for all your drives and that it is well ventilated with plenty of fans to cool the system.
The next, and often most overlooked, part to consider is the power supply. The power supply is, of course, what powers everything in the computer and without a good power supply to deliver consistent, even voltage to everything at all times you could have random crashes, lockups, and even loss of data. As far as wattage goes you should have absolutely no less than 300 watts and probably should go with anywhere from 400-500 watts depending on if you have multiple processors, several drives in RAID, etc. One other thing to note is most mid to high-end servers have redundant power supplies in case one fails you have a backup to keep the system running. These can be, and usually are, hot-swappable so the server can remain online at all times even while swapping power supplies.
The motherboard, which rounds out the foundation, is a pivotal choice since it dictates expansion capabilities, and is basically the heart of your computer. Without a good motherboard your server is practically worthless, plain and simple. No skimping should be done here. And it's not so much a question of features or performance as it is stability.