If you go to your bank's website, they should have a loan payment calculator. Just punch in the figures, and it will give you the payment or whatever else you are looking for.
If your bank doesn't have this on thier site,
www.compassbank.com has a good one. Check under the consumer lending section, and there is a calculator there.
Obviously the more you put down, whether it be cash or trade in, the less the monthly will be.
Don't forget about sales and excise tax though, on a 50k vehicle, those aren't going to be cheap.