I'll tell you right now that $3-400 won't get you too much unless you have some nice hook-ups. But just to let you know ahead of times, if you ever decide to change your head unit, your sound will be worse then where it is right now unless you also change your amp...in which case it will sound much better. As for subs, Depending on the sound you are looking for, I would recommend a 12" sub b/c you need the surface area to get a wider range of frequency. Those low bases need a large surface area. If you can, I highly recommend Diamond Audio TDX subs. You can never ever go wrong with them as they are one of the best. But since you are trying to stay on a budget I would try the Diamond Audio M5..or was that the M6...which is the next step down. But don't get me wrong those are still some amazing subs. Not only do they sound great, but till this day my friend is having a problem trying to blow them out so that he can get new ones...lol...but it goes to show the quality of the subs.
As for amps, again, the Diamond amps are also great. But if not try the alpine amps.