firstly, whatever it says about watts is meaningless unless it is watts RMS. you cant compare other figures. my sub is 400w RMS but 1600 watts "max". i could find another sub that is 1600w max, but only 250w RMS. there's a big difference but both would say "1600 watts" in big writing on the front. that means nothing.
to decide what to get you need to know what your long term plans are.
if (like most people) you intend to stick some 2 or 3 way 6x9s in the back and some 2 or 3 way speakers in the front, and power them all from the stereo, then you dont need much bass. a simple 100w RMS 10" sub in a small enclosed box would be best, and an amp giving less than 100w RMS. then you'll have plenty of bass and still be able to use your boot. as always, good quality gear is better than cheap stuff that claims to be twice as good.
if you intend to leave the original speakers in (stongly advise you dont on most cars) then you wont need much bass. any old sub and amp will probably be more than enough to drown out the original speakers. a cheapy sub in a box with any old amp will do well enough in this type of situation, lots of epolpe do it, but i cant see the point.
if you have plans to fit decent components front and rear and power them from an amp, you'll need a good bit of bass to balance the system out. in that case you want a 'mono' amp and a sub with at least 400w RMS, a twin voice coil sub if possible, and a large ported box. this will give stong wide bass, but it isnt as tight or as accurate as having a smaller enclosed box. it depends on your preferences. i'd go with a 1ohm stable setup like this if i had long term plans to create a great system.