Converting a car to natural gas is a pretty big undertaking. Your now storing a compressed gas, and moving a compressed gas. As far as I know, even though natural gas burns cleaner, it does not provide the same power as gas, so you would have to carry and burn more of it to get the same mileage and performance.
E85 is an involved conversion too, but I think it is a lot easier to do than a natural gas conversion. It does not provide the same power or mileage as gas either (but it is a better alternative fuel source than natrual gas), but Team Corvette is running E85, and ALMS allows them to carry extra fuel to make up for the lack mileage.
In short, I would consider buying a purpose built Flex Fuel vehicle, but I would probably not convert a standard gas powered car to E85 with out a lot of research.
Jason