It depends on how much money you want to spend and what you are looking for.
If you want just a nice clean bass without ripping your head off, just get one 10" sub in a ported box and a small amp.
As far as what amps and what subs? I'd suggest any high end brand, you really can't go wrong with any of them.
Eclipse, JL, Rockford, Alpine, Sony ES
Just stay away from the cheaper stuff--JVC, low end Kenwood, Sony, etc.
If you want more advice, just pm me, I have lots of experience in car stereo and can help you with installation questions!!