If you're already into negative equity on the IS300, the last thing you should do is make things worse by rolling that debt into a $500/mo payment on a new car.

The Si isn't going to save you much on gas because you'll be wringing it's little DOHC neck all the time.

If the IS300 is running strong, keep it and keep up on the basic maintenance. It's a very nice car with responsive handling and decent room. Do all that you can to pay that car off ASAP.

Also, what's the APR on your current loan?
If you've got good credit, you may be able to refinance the loan at a lower rate.