I setup bind on a 486 DX2 with 48MB of RAM for the company I used to work for. Still running right now.
The company was given five IPs by their Telco/ISP. I set the websever and the DNS server to answer on the same IP, but probably not the best setup, security wise. Didn't have much choice though; small company, small amount of cash. Hey. It was all running on a 486. What do you expect?
Just get a router, maybe linux based? IPCOP, FREESCO anyone? Then set it up to forward the ports for DNS to the IP you have.
If secondary/redundant DNS services are what you are worried about, setup an account with DynDNS.org. I did it for my previous employer and it only cost US$14.95 a year, IIRC.