Sup Deano....
For 300bucks you would be surprised how much you can accomplish.
First things first....do you have an aftermarket Head Unit?
If you do then you are good to go and all you will need is a decent amp kit, amp, and sub(s) + sub box.
I agree with rick about getting one nice 12 instead of a couple just because of your budget.
One thing you need to think about is space.....how much space can you use in your specific vehicle? If you have the space, a nice 12 ported would probably suffice.
Also, once you choose subs, you need to know what your final Ohm load will be to choose the right amp. A dual 4ohm sub would match up to most Small Mono amps that are stable at 2ohm mono. A single 4ohm sub would match up well with a 2ch amp that is stable at 4ohm bridged.
For really cheap amps, one of the first place I check is
http://profilecaraudio.com/amplifiers.htm They are cheap but you will get decent power and decent build quality. "alot better than getting some cheap *** Dual amp from BB or Walmart.
For cheap subwoofers I would check these links....
http://Http://edesignaudio.com one 13Kv2 would sound alright with a good 300-400Wrms
http://www.soundsolutionsaudio.com/shop/index.php?act=viewProd&productId=205
Their are a bunch of setups you could go with honestly, but hopefully these links will get you started on your comparison shopping
curtis