Choosing a Firewall

Just so happens I watched a very impressive demo this morning at:
http://www.zonelabs.com/store/content/company/corpsales/whitepapers.jsp#
Enterprise Firewall called Integrity from Zonelabs. There is a graphics button to view demo at about 2pm.

For a business network, I recommend BOTH a hardware and a software firewall.

Zone Labs Enterprise version is likely a very good candidate for a software firewall.

Take a look at your routers and switches.
As a general rule, I would add a firewall appliance from the same vendor as the majority of your network equipment.

Chas

Take a look at Microsoft ISA Server. It's an Application Layer firewall. It also has the capability to "publish" the web email and ftp services that you host. A good secure setup would be to have the ISA server set up behind a conventional stateful packet inspection firewall.
Having an application layer firewall is becoming
increasingly important with modern threats.
Take a look at isaserver.org
I recommend Shinder's book ISA server and beyond for an ISA Server topology strategy

Try the SANS Reading Room: Firewalls for some good reading information. Things like what platforms you're running, what remote access is required, etc. become important with a choice like this. There is all sorts of information that may be useful here
http://www.sans.org/rr/catindex.php?cat_id=21

There are many solutions available. Some that I have used are Cisco PIX, SonicWall, Checkpoint.

You will want a good one to support your NAT function as well as the possibility of VPN.