How much do you drive annually?
Did you buy your Intrepid from a dealer or a private party?
Dealers always have a pretty hefty markup so you'll probably get way less car for a $4500 trade-in, maybe something in the $2500-3500 range.You might be able to find a toyota echo that gets 33mpg or so, and that would save you about 200 gallons per 10,000 miles, or $600-$1000 per 10k depending on how gas prices go. Still that could also will have lower maintenance costs.
But inventory at a single dealer is going to be very limited and I imagine you'll be picking over some 25mpg cars with higher mileage and questionable maintenance, which probably won't be worth it. You might have more success selling the Intrepid and then buying a new car on craigslist.