It also depends on what kinda music you listen to, what kind of money you want to spend, and if you are going to do it yourself or let a shop do it for you.
I believe some shops will hook a bass tube up real quick and dirty to let you hear how it sounds.
If you really like to play music in the truck and want it loud, you will eventually want to get a real sub with a box. You can definitely pick up a sub + box + amp of decent quality for 300 bucks or under. This will add lots of oomph to your system. Of course if you listen to country music or something else without a lot of bass you will definitely want to go ahead and replace your separates before you get a sub.
Do some legwork and get into some of the custom sound shops in your area. Most of them will be happy to spend some time with you and let you listen in on the differences.
Just like going fast, you can always make it better with more money.
ps: when you buy more than $400 bucks worth of stereo equipment make SURE to get an alarm on your truck. I've had to learn this the hard way. Theives break the little vent glass, open the door, take all your stuff, and are gone in mere seconds. Anything you can do to discourage them is worthwhile.