www.johntimney.com

This is a massively complex topic and one you normally need a lot of experience to attempt to do and expect to have a credible answer.  However, MVP Cornelius van Dyke has put in a  lot of legwork to create an online tool to help with this.

You can find it here:

http://www.cjvandyk.com/blog/Lists/Posts/Post.aspx?ID=100

If you feel like doing it the hard way then the HP sizing guide is what you need.

You can find that here:

http://h71019.www7.hp.com/ActiveAnswers/cache/70675-0-0-0-121.html