Hmmm... I'm doing the same math myself.
I get about 20 MPG now, and I drive about 9000 miles per year, so that's $1,350 a year at $3.00/gallon or $1,800 a year at $4.00/gallon (where it seems to be headed in CO).
Assuming I don't drive any less (or more), and gas goes to and stays at $4.00/gallon, I could save $600 a year by upgrading to a car that gets 30 MPG; or nearly $800 a year if I get a car with 35 MPG.
Assuming gas will get increasingly more expensive over the next 10 years, it makes sense to me to get a car with a really high MPG now, and drive it for as long as I can. If I averaged a savings of $800 per year over the next 10 years, that'd be awesome.