paldo,
The dsl providers in my area allow multiple IPs to be used. Right now they say they aren't limiting them. I figured it is always worth a phone call to see if the easiest solution is possible.
Another solution, if you have an old 386 or better machine lying around, is to get 2 network cards and a copy of a disk image of linux router project. Search for "linux router project" for info. It is free, doesn't require a harddrive and gives you a dedicated gateway for your network.
ttn1