Get a nice head unit. These are the things most people overlook. A good head unit with a build in bass crossover is always essential for amazing sounding clarity and good bass.
Although one subwoofer would do, I'm a bass lover.. So I'd be more prone to go for 2 12" Subwoofers.
As for your question on amps, well, you can't really have an operating sound system without an amp. You NEED one. These are what the subs hook into and draw their power from...
A good amp would be something around 500 watts or more.
500 watt amps aren't pulse pounding, but they do sound nice. Although, if you want to go the more expensive route, I'd say get an 800-1000 watt amp.
If you're getting two subwoofers, I'd say make sure they're around 800 watts each as well. But remember, although it is possible to have a 500 watt amp connected powering two 800 watt subs, that's 1600 watts total drawing from only a 500 watt power source. So you're cancelling yourself out there. This simply means you wont be getting the full power for what you actually paid for because the subs are trying to draw more power from what isnt there.
I'd get something like this
200-400 dollar head unit with built in crossover
2 12" 800-1000 watt subs
1 1000 watt amp.
This will literally make your car pound.
However if you want something less intensive on bass and expensive in your pocket, I'd get something like this
200-400 dollar head unit with built in crossover
2 12" 500-800 watt subs
1 600-800 watt amp.
Remember this as well: If you want the best sounding quality, it's recommended that you keep all the brand names intact throughout your system.. Meaning, if you have a Sony brand sub, get a sony brand amp and head unit. Don't criss - cross over like having a panasonic head unit, sony subs, and an MtX amp.
It doesnt mean your system will sound like crap, it'll still sound amazing and perfectly clear, but it's hard to explain.. Listen to a system with the criss-crossed brand names, and listen to one with all the same brand sound system.. It's much different.
As well, I don't find that brand names really mean much, they all pretty much do the same thing. It's like comparing a 27 inch Sony tv to a 27 inch panasonic tv.. There's really no difference in quality I find, but some may differ. Either way, it'll do the same thing.