It is important to have an up to date inventory of your hardware and software. Knowing what you have can help you decide what to buy and when to buy it.
When you get new equipment be sure to update your documentation immeditately. It is much easier to do then.
Here are some good resources for starting your inventory:
www.techatlas.org Tech Atlas has a great online tool.
www.techsoup.org has a good worksheet at: http://www.techsoup.org/learningcenter/techplan/page4903.cfm?cg=searchte...