Listen to some JL subs. I have a single 12" W3 and most people assume I'm running 2 10" subs in the trunk. It's a damn nice sub.
You haven't mentioned what type of music you listen to. If you want sound quality as a top priority, shoot for a sealed enclosure stuffed with polyfill. It will give you nice, tight, punchy bass. If you want god-awful loud sounding music, then go for a ported enclosure.
Personally, if you want to stay on the cheap side of things, get a pair of JL W0 (zero) subs. They take next to no power to get loud and have decent sound quality. I personally feel the W3v2 that I have is the best price😛erformance/SQ ratio in the JL lineup. Get a W6 or W7 if you feel like unloading your wallet a bit, but treat your ears to some sex. 😛 I run my single W3 on an Alpine M501 550W@2 ohms and it's really too much power for it, IMHO. A 350W amp will do the trick.
BTW, I have heard good things about RE Audio's XXX series subs. They're comparable to the W7 series from JL, but I've heard they get a bit louder. (be prepared to juice these suckers up with lots of amp power)
Just don't, and I repeat *DON'T* go to Best Buy to get your stuff. Buy online, save a lot and get something that sounds good. The only thing BB has going for them is their Alpine head units.