[z28dreams]

Used is generally good advice, but not always entirely true.

With some new-car incentives, rebates, etc, an 05 or 06 model could cost almost the same as an 07 model. My parents ended up going with a base '07 trailblazer for this very reason.

BigBootch hit the nail on the head - here's how it typically works:
first 2-3 years: heavy depreciation
next 5 years: all depreciate around the same rate
after that: very little depreciation

You can drive something like a 1996-1998 model car now for a year and probably almost break even selling it if you're good negotiating.

Also, don't get a sentra. Get something you'll want to drive for a while.

One major mistake people make is to buy something cheap because they don't want to spend a lot of money. In 2-3 years they get sick of it, and end up selling it for a big loss and buying something new.

Try to pick a car that's nice (but not so nice you can't afford it), and try to drive it to the ground.

For around $6k, you should be able to find some decent cars.
For example, if you like nissan (since you mentioned the sentra), you could find this car:

2000 Maxima SE
75,000 miles
Leather
$6500

(edmunds TMV, private party, average condition).

This would be a MUCH nicer car than a sentra.