It depends. You need to know what their car is worth and what your trade-in is worth. BTW, you'll have to pay down your current loan first, unless your car is worth more than $8000 as a trade-in.
Go to the dealer and make an offer. Don't stay for more than a few minutes. The dealer will try to wear you down. Give them your name & phone number, and leave. Do your negotiations over the phone, NOT in their showroom. Don't use dealer financing. They will likely try to overcharge you. Get your own financing from a bank of your choice.
How to Negotiate a New Car Price Effectively
http://www.consumerreports.org/cro/2012/12/how-to-negotiate-effectively/index.htm