Ah ok you haven't bought anything yet.
Well, choosing what sub to get depends on the amp you have. Have you bought an amp yet?
Check out the Dual Voice Coil wiring guides and tutorials to see what subs to use to match up to what ohm load the amp is safe to run at.
http://mobile.jlaudio.com/support_pages.php?page_id=127
I prefer to use two channel amps and bridge them to run a 4 ohm load. So if I was getting one sub I would get a Single Voice Coil 4 ohm sub or a Dual 2 ohm voice coil sub.
If you chose to use more than one, check the charts posted in the link.
If I was gonna buy a sub these would be my top choices on a budget
http://www.tcsounds.com/tc1000.htm TC1000 12" sub on sale for 159
http://www.soundsplinter.com/rli_ign...r_drivers.html
Or one of the Dayton Titanic or Reference subwoofers.
http://www.partsexpress.com/webpage....822963#drivers
For amps, I'm liking the Boston GT series amps on ebay. They're nice amps, low profile, and produced in Italy.