Total n00b: startup help


Firstly I want to apologise if Im posting in the wrong place, or this question has been asked 1k times - fact is Ive been keeping an eye on this forum and website looking for information for some time but haven’t been sucessful in finding answers to my questions.

Say, I want to setup a voip solution in our office, at this moment we are 4 people, and recieve around 100-150[1] calls everyday from several countries around the world.

I have some hands-on knowhow using Gnu/linux, so setting up a linux box for this is not a problem.
We already have an Alcatel PBX[2] in the office as well alcatel handsets.

For the voip solution, I im thinking about a dedicated DSL 2Mbit/512kbps line or 2Mbit/768kbps
Can we have a faily good sound quality with this type of bandwithd ?

What do I need to do next ? - I asume I need to buy a voip number. I’ll sort this out later on, but please post if you can recomend some.

Which special hardware do I need to purchase for the linux box running asterisk ?
Say a normal P III 1000Mhz with 512MB ram, nic adapter, and 80Gb disk is enought ?

Whats the hardware requirements for 4 people talking at the same time with customers?
( let add som room for growth - so lets calculate with 8 )

How do I connect the whole thing together ?

And how can the users use some software in their pc’s to control the calls ? like forward to another number, put on hold etc…

[1] 100-150 calls a day its just a guess, we dont have any statisk program.

[2] I dont have at this moment more details about the alcatel pbx or the handsets, but can provide model numbers if its necessary to answer the question to wheather it is usable or not.

Thank you very much for reading my questions and taking the time to help me.