Colin, if your choice of cars is limited to those that emit the least amount of pollution, then buy any car with a ULEV rating. There are lots, and they are all gasoline engines. The old adage that diesels are cleaner than gas engines is no longer true, and hasn't been for years. Talk to the EPA.
As to using bio fuels to augment diesel, or to run bio fuel exclusively, here are the numbers. In North America around 145 million gallons of gasoline is consumed daily, about 44 million of diesel. At most, vegetable oils are capable of adding around 600,000 gallons per day, and that is assuming an infrastructure that can deliver that amount routinely. This is highly unlikely in the near or long term. Consequently I fear that the real reason behind using biodiesel is guilt, not science, based.