I'd get a new head unit first. That'll be about $250 installed and you can use the last $50 for new front or rear coaxials off of ebay. Maybe some bostons or if infinity.
You should be able to install the coaxials yourself (for free) if you can do basic car/truck repair and you got a 6x8 coax meant to replace ford speakers. You just pop off a panel mess with a few screws, a few wires and you're done. If you put the coaxials up front, you'll want to move them to the back when you save up for the next step.
Next step is to save up for some nice seperates and put them up front. Go for the same brand as your coaxials that are in the back (or will be moved to the back), just because same brands often (but not always) sound better together. I'll explain why if you need me to later. You can get a small tweeter and a 6 1/2 inch mid in the front door if you or an installer builds a mounting plate/adapter. This step is about $150 to $200 for the speakers and hopefully you can get the install done for about $75 to $100.
After this is all done, it's finally time for the amp and sub(s). You may want to look at a nice 5 channel amp if you want nice loud (but not thunderous) bass. It will run a 10" or 12" and all your mids and highs. Alpine made a nice 5 channel at one point. My friend got one of the net for about $200 or $250 if I remember correctly. Amp install kit will be about $50 and labor will be around $75 to $150 depending on the quality of work... Sub and sub box will be about $150 if you go with one sub and a prefab box. This is where a ton of money can be spent on custom box work though. Stealth boxes and fancy boxes can run several hundred dollars.
If you want tons of bass, get a seperate amp just for the subs and go with two twelves or four tens. Thunderous bass takes more power and more speakers, so that means more money. My wife had one ten under the rear seat and it sounded real good, but couldn't be heard from the car next to her. I had a friend with two twelves and it COULD be heard from the car next to him.
And you can always skip the mids/highs in the doors, but I would suggest you follow this route. You'll be surprised at how much more fun your stereo is each time you go to the next step if you follow this plan.