$300...okay....this is ASSUMING you are not going to HU route...
You can either:
1) splurge on a GREAT 2.1 system
2) get a 4.1 system, and still make it great, but not AS great
ASk 3onDubs, he did some goodness with $300. I did my goodnes siwth $150 then said screw it all. Try to spend $250 on components and the rest on wiring, unless they let you take some home from teh BB install bay.
If you want loud and clear, and not too much bass in the speakers, which keeps it in the subs, where it needs to be, check out Infinity Kappas. CDTs and REs are also good and you can check out Sounddomain.com for their classifieds. Try to get them between $50-$80, especially if you're getting 4 speakers. Try not to cross over $125 for all 4.
Here's why: the amp and subs. Kappa Perfect, ID, Kicker Solobaric. You can get them for cheap (don't go over $70) and get ONE 12"! Done right, you will get all the bass you need. However, done right means you need a good box. Spend $30 and BUILD YOUR OWN. You'll learn lots in doing it and have some pride in your system.
As for the amp, you have ~$75 left. if you have a 2.1 system, get a 2 or 4 channel amp. if you have a 4 speaker system, go with 4. I'd go 4 incase of future upgrades, regardless. Make sure you have everything speced out correctly in terms of watts and ohms. If you can spare a few extra bucks, try to go with the best amp you can find. best advice i can give you. That or jack a car with a system you like