The right server set up for collaboration and email

Well a server class linux/unix server for your MySQL database.
The problem with your question is that it tries to combine many things.

I.e. document collaboration may mean that you would build a Document management system. There are several open source ones.
Here you can define groups that will have different access to different documents.
Email can be open source unix/linux or mercury mail for windows.
 
The issue is more with what administrative capabilities exist in your environment. I.e. if you are open to using different OS's to get to where you need to be or if you are looking at a single platform to achieve your goal, that will narrow down the possibilities.

What is the environment under which this fanctionality needs to be made available?
I.e. a windows AD with primarily windows clients.
Do you want this collboration to be web based? Or web based collaboration is out of the question. etc.

You have to narrow down the parameters.
The hardware for the server/s will depend on the number of users, amount of data, etc.

NOt really sure what network connection you referencing, i.e. two load-balanced/fail-over connection just in case one network interface goes down because the switch to which it is connected goes bad?

Ah yes, thanks arnold

Well, all the client machines are either running windows XP or Vista. There are only 6 clients in total.

In that circumstance, is it best to run Server 2008 and configure MySQL & PHP for use with IIS?

Great, thanks very much Arnold.

it would probably be much easier to just install small business server with exchange and sharepoint.
that would give you mail / groups / data sharing / webmail
you can easily install a mysql on it or use mssql...
This is very stable if your hardware is functioning and you make sure you update the machine regularly and have a propper firewall setup between the server and the internet.
greets

RightNL is right on the initial part.  the problem is down the line as the use grows it becomes very expensive to transition to a new setup or to maintain the same infrastructure.
Unless you have a vendor required OS/database server, looking at open source solution could be less expensive in terms of initial money outlay while requiring administrative work hours (on going smaller amounts of money outlays).

make sure to setup a backup plan and look/consider a disaster recovery plan.
I.e. what if x, what happens?  This type of planing will prevent a massive headache and expense down the line.

transition from 1 small business to another isn't that hard..
If you expect this company to grow you might be better off with the non small business although small business does support up to 100 or so users. and you can use member servers to scale functionality.
I heard that in windows 2008 there's a better upgrade path and there is also a medium business version but I haven't worked with it.
greets..