might be a silly question but....

I think going with one higher priced sub is allways better than going with 2 subs that cost half as much (to an extent). +1 on checking the RMS rating. I have heard that you need to worry more about blowing a sub with an underpowered amp though than with an overpowered amp because the distortion would kill it. I've also heard that its very hard to actually 'blow' a sub since a quality one (over $50) is usually made pretty sturdy. The one sub I had that failed was because it was a cheap one and the glue that held the paper to the cone separated, not from exessive power, but because it melted from being in my '92 hatch face up in the Texas summer with no tint.

It also helps to buy dual voice coil if you can. So say for instance, instead of a regular 4 ohm sub, buy a dvc sub with 8 ohms to each coil, then wire it in parralel (or is it series, someone else confirm) so that you get 4 ohms total. This doesn't give you any real gain in sound quality, but it adds to your flexibility to reuse the sub in different applications. For instance, if you decided to upgrade it later on, you could build a suitable home subwoofer and use an old 2 channel home amp to drive the dvc sub. I used to do this and it was pretty cool.

This is just my limited knowlege. Take it with a grain of salt because I haven't even had a system in my car for over 2 years.

Go to your local best buy...
Walk into their car audio section.
Go into the "sub" section.
They should let you pick which subs, with which amp, with which 6x9 component speakers. For the audio illiterate, its a place to stand on.
All at the push of big red buttons.
They encourage you to turn it up too...

You can also from the main switch panel go thru demos of each type of music to see which ones suit what type of music.

Me and my woman say, Rockford mono amp.
Two puch 10's, vented box
punch 6x9's.
Now I tried a single punch 12, a dual box with 2 10's, power acoustik <-- can't spell that. And misc amps and configs.
The 2 channel Rockford amp was nicer, but not by THAT much, and still let some background noise in. The mono gave you the thump, without trying to play the music at all. It hit cleaner too. Not to mention was $400 cheaper.
The power acoustik stuff was all garbage. The punch 6x9's sounded better by themself. I'm NOT a stereo guru either. Luckily my buddy who used to do db comp, will install, and supply whatever I want.
I know JL audio stuff is great too. If I bought a single sub, it would be JL.
With a glass hatch back to bounce it off of...
It won't take much. Go for clean sound. The car will do the rest.
I've been in a CRX that could bang Rammstein hard enough to stand your hair on end. It felt like it would push you thru the windshield. Hatch back.
My friend ran a 6 12 sub wall in the back of a 97 neon. Neodome horn tweeters, 2 full component sets, plexi windows, and you couldn't even be in the car when he turned it UP UP. It would burst your damn head.
I let him do my audio.
Good luck!