Asterisk for the buis

We are currently converting the office to voip to save on phone costs and reap the advantages of voip my boss tells me. I have been assigned to research a telephone solution that will give the business a professional feel. Today I came across Asterisk and I like what I see. After watching the system explanation on utube I am pumped and cant wait to dive right in. I have currently pieced together a linux box for around $200. Case, mother board + cpu w/ integrated sound and video and onboard lan, 40 gig hd, and cd rom. From this point on I am a little lost and I am not sure if I need a special card or not. I also need to upgrade the phones and it seems like there are so many options. To better understand what we are trying to accomplish I work for a small company with like 6 people. We want a more professional feel when people call in. Basically we want a caller to be greeted with a welcome message and then be able to choose through some menu options or dial an extension of a person and if the person doesn?t pick up then they will be sent to their voice mail. Now i know that all this is possible with Asterisk and so I need help to make sure that I get all the right equipment. I am insure of how voip works and so i wonder if we need more that one voip line to talk to a customer and have more in queue and at the same time be able to dial out? Also I wonder what kind of internet upstream do I need to make all this work? Sorry for the long post and thank you for any help.