Proxy server requirements

I want to add a computer that our company internet connection would connect to. It would have two nics and one would go to the router that connect to the internet and one would connect to our LAN. I want to run a different brand of Virus software(different than what we run on our net work)I may also want to run maybe Proxy server and our some type of intrution detecting. What would the requirements for this be, like OS (would windows 2000 pro?) what memory, Nics, Disk space?. I have put this question at higher point so I can accept multiple answers