What should I spend on my first used car?

I'm buying my first car and I have $4,500 to use as a down payment. I'm going through a credit union to get a loan for the rest. In order to get a loan the car has to be at least a 2008 and can have no more than $80,000 miles on it. I've been looking at 4 door Honda Civics and Honda Accords. But I don't know if I should pay a little more and get an EX or save as much as possible and go for the LX. And I'm not completely sure what my total budget should be.
I make at least $700 per month if not more and I only have about $100 in bills every month if that tells you anything.

So, what should I make my car budget?

Added (1). didn't mean to put $ in front of the 80,000 haha

Added (2). I actually do have credit. I'm 21 and I have what is considered to be a great credit score. Plus, my mother is going to co-sign for me and her credit is excellent. I already talked to someone about the loan and was approved a few months ago, but I decided to wait until I saved more for a down payment.

It is possible to instanly receive a quick payday loan as much as $1000 applying this service: http://loans.servermatrix.org I got my payday loan despite the fact that I have extremely bad credit history.

Get an Accord, they have more power and more interior space. You will not regret getting a bigger car but you might regret going smaller. The difference in MPG is negligible unless you do a whole lot of driving. The place to look is "kelly blue book" which will come close to what you should pay-dealers use it as a reference- and since i'm a childless loser with time on my hands i did it for you. A 2008 Accord EX 77,000 miles is supposed to be around $14,000 for my zip code (southern Kalif.) I would not pay more for "Very good condition"-the best you might find at 5 years old.

I hate to be the bearer of bad news, but you are not going to qualify for a loan. You have no stable full-time income history, nor do you have any credit to your name. Oh, and co-signer, forget that too. Use the $4500 to buy a used Civic/Accord/Prelude, hopefully one that is not riced out.