First step is to pick a vendor that you are comfortable with. I know this is different from what most consultants say. The new server offerings are really very similar, both in capability and price. The difference really is supportability. I have worked in both IBM and Compaq shops (very large installs). I recommend the compaq, but if you are more comfortable with the Dell or IBM folks, go that way.
I recommend that you select what your expected server life is, and plan on what you need then. National average for data growth was 40%/year the last time I looked. Plan on that as your factor.
Single processor would work, dual is better. Get 1024mb of RAM per CPU. Definately plan on RAID-5, and a chassis with room to grow (a 1-U chassis may work now, but has no room to grow. Don't forget a way to back it all up.
Rich