router vs firewall?

I just got an upgrade for my cable broadband with a static IP address from Comcast. Belatedly I found out that they turn the firewall of their router off once your got a static. I do have a ZoneAlarm which seems to be doing a good job but people in the know do recommend a hardware firewall installed too. I called Comcast and they recommended not to assign the static IP address to the computer but instead to a router which will be put in between the modem and the machine. I have had such configurations before. A few months ago I got rid of three Linksys and Dlink routers I accumulated over the years since I thought that I would never need them again once I got a professional broadband with modem/router built in. Now I am facing a few choices.

I can buy another Linksys router but in the past I came to mistrust them badly.

I can buy a brand new Cisco, perhaps 3800 series but they are quite expensive and offer much more than I need in terms of LAN capabilities.

I do have two workstations with Win2K at two different locations with two computers at each one. I am using DELL Gigabit switches for the LANs. I have some peripherals and I am planning to set up VPN and I need a full capability VoIP solution. I want a remote management too. I do use Vonage telephones (VoIP) at both locations.

I can purchase an old Cisco at eBay, perhaps 2600 series but they are outdated, frequently lack documentation and do not offer modern capabilities.

I feel that I should not spend more than $600-800.

I am thinking of purchasing a Cisco firewall as a stand alone product. They run in a few hundred dollars. One option is a Cisco PIX 506E Firewall used. Any other suggestions?

My question is: will a Cisco firewall work with a non-Cisco modem (Comcast)? What will be disadvantages of using it?

Thanks.