Question : VOIP Systems for small businesses

I'm in the computer business supporting small businesses. I have never used, installed, or maintained any kind of VOIP system. I'm familiar with the basic concepts but I do have several questions. If you can only answer one question or give some small suggestion, please do it. I don't expect anyone to write a 40 page "answer" to my questions. Some links to other sites would be OK too.

I'm considering VOIP for some of my clients, first, because they are asking about it, and second, because I assume VOIP can give us all the features of a PBX system (if not more) and it will be more affordable.

Are there any good affordable solutions for businesses that have 2 to 10 employees? Something in the $1000 to $4000US range?

I'm willing to piece stuff together if needed. It doesn't have to be a complete system, although that would be nice.

I suppose I will need some kind of service provider, some software and/or VOIP routers, and some kind of hardware/phone setup. Any recommendations on these?

How can I know if the clients current Internet connection provides enough bandwidth?

Here are a few of the features we'd like to gain by using VOIP:
Auto Attendant
Voice mail boxes
Conferencing
Call forwarding options
Intercom
Call Recording
Call Logging
Ability to extract CallerID from incoming calls (for use in our MS Access Database)
Cordless long-range handsets
Pre-recorded informative messages for callers who want general information

Answer : VOIP Systems for small businesses

Check out Asterisk.
  http://www.asterisk.org/about

or scan the wiki for a ton of information
http://www.voip-info.org/wiki-Asterisk

Also highly recommend this book for not only asterisk info but general voip info that is easy to understand,  you can buy it or just download the PDF
http://www.voip-info.org/wiki/view/Asterisk%3A+The+Future+of+Telephony

There are quite a few turnkey solutions based on Asterisk if you want something that is ready to go. Here's a couple of examples,

http://www.fonality.com/pbxtra_features.html

http://www.trixbox.org/index.php

Asterisk also has multiple vendor support for hardware to work with both digital lines or old analog.  So you can simply hook up to existing analog lines AND also connect to most voip providers.  Kind of the best of both worlds.

Asterisk for a small setup is not demanding so you would be looking at using any current PC plus line interface/station interface hardware.  Right now its around $100 per analog port - so each line and if you re-use analog handsets.  New voip stype digital phones do not require any hardware ports though they do cost more than a analog set.

Grandstream make a pretty decent digital business phone,
http://www.grandstream.com/y-gxp2000.htm  

The biggest thing going for Asterisk is there is not much it cannot do.  It scales right down to single line home phone setup and right up to a large call center with multiple PRI trunks.  Also works with pretty much any voip phone/service out there.

Don't let the setup scare you either.  Though there are a ton of features and settings but you probably won't ever need to use more than a fraction of them for a basic office setup.  The pay turnkey style setups are mostly all gui driven so even easier for somebody to setup/modify the system.