Yup, I worked at Best Buy years ago. If you know someone there, you can get great deals on stuff.
Fly Girl, you said that your sub went out, so that means you have the JBL system. The speakers that you have in the doors are not bad for a factory system (they're actually sperates - 4" mid and 1" tweeter), so with a $500 budget I'd leave them alone and concentrate on the low end. Keeping in mind possible upgrades, this is what I'd do.
For now, run the door speakers off the head unit. A seperate amp for them and some new speakers can come as your budget allows. Get a dedicated mono amp to power the subs with a built-in crossover, so you can filter out the highs from the sub.
Subs: Right now,
www.woofersetc.com is having a sale on Nakamichi 12" subs for $89 each. This is a fantastic price on some very well-regarded subs. To be honest though, unless you want to really pound, I don't think you're going to necessarily need two twelves. I have a single 10" a/d/s/ sub in mine, and it sounds awesome. In fact, with your budget, a single quality twelve should easily outperform two less expensive subs. A great choice would be an Image Dynamics IDQ12V2. Woofersetc has them for $185 each. The Alpine Type Rs are supposed to be awesome too. Woofersetc has them for $145 each.
Amps: Again, a mono block is a great way to go. The JL Audio 250/1 amp should power that IDQ nicely. The new JL amps are supposed to be awesome and Woofersetc has it for $289. The new JBL amps are supposed to be great too. The P180.2 would put 360 watts to that IDQ. Woofersetc has it for $190.
Personally, this is what I'd recommend: One IDQ sub with the JBL amp. The IDQs are rated for 350 watts RMS, so the amp would be perfect. Both products are well-regarded, sound great and are virtually bulletproof. Plus you'll have $125 left in your budget for the box and RCA cables and whatnot. I bought my a/d/s/ sub from woofersetc and was very happy with their service, plus they're authorized, so you get the factory warranties. Good luck!
Mike