i think its a safe bet to say, that unless u have a luxury model car, any aftermarket system is going to be way way better than ur stock system. Depends on how good u want it thats all.
i have a friend in a similar position to boners, bought some splits, some 7x10's and an alpine type S sub, amped the splits and sub with a boss audio amp, and it sounded very very nice! Then, satisfied wit his system, he spent his money on other things.