4 cyl? dvd/cd player first by far. i've got a pioneer avic d3 and i love it. i highly recommend their products. lots of info on how to install their stuff on here too which helps. also AVIC411.com if you have any questions about the system that the users here don't know
I am firmly in the camp to save your money right now. The economy is still too iffy for me. All but one of the economic news letters I get say there will be another downturn before things finally do recover.
On the other hand if you feel like you have to spend some of it I would vote for the exhaust system first. You probably already have a cd player in your truck and unless you are one of those people who is not happy unless everyone else can hear what you are playing then the factory unit will probably suffice.
I usually listen to talk radio when I am in my truck so I only listen to cd's when there is nothing on the radio I want to listen to.
By the way, how do you know for sure you are going to get a return?