I spent just about $2k on my car system back when i was 18. Since then its been moved to 4 different vehicles in the last 5 years. It still works well, but it is by no means as cool as it was when i was 18. Plus i have no trunk space. I hope to be getting a new GTO (if pontiac ever releases them) within the next couple of years, hopefully the stock system will be good enuf. My friend and i went and test drove a saturn vue this weekend. It comes stock with an 8in sub, i was pretty impressed.
if i were you, i'd just throw a decent cd player in it, get some decent all around speakers and put 150watts behind them, get some bass blockers for the front and it shouldn't sound all that bad on the inside. An 8in sub will make the lows more defined, but that might also sacrifice some room. Crutchfield is a bit more expensive than just ordering from the net, but they'll hook you up with diagrams and charts showing you how to take your car apart. A lot of times tho, installation is free at best buy type car audio places if you spend a certain amount of money.
ed