it depends on how much you are willing to spend and how much you want to upgrade
if you are serious but don't want to break to bank or have to upgrade you interior speakers (from being drowned out by the bass) then i would suggest a couple of 12s with at least 400watts RMS total in a nice ported box
always listen before you buy and pick what sounds good to YOU
when it comes to car audio nine times out of ten you get what you pay for- and you need to look for CEA-compliant brands which means that the amps have been tested and produce or exceed the advertised RMS wattage with minimum distortion and the subs can handle their advertised RMS wattage also (distortion is the main cause of speaker damage over time)
here are the top CEA-compliant brands
Alpine (highly recommend)
Bazooka (don't like their subs)
Blaupunkt (don't like their subs)
Clarion
Eclipse
Infinity (highly recommend)
JBL
JL Audio (very good but over priced)
JVC
Kenwood (don't like their subs)
Kicker (the best in my opinion)
Lightning Audio (have not heard their subs)
MTX (highly recommend)
Pioneer
Polk Audio (highly recommend)
PPI
Rockford Fosgate (highly recommend)
Sony (don't like their subs)
you want at least 400watts RMS if you are serious (a little over $500)
http://www.sonicelectronix.com/item_3867_Kicker+DCVR122.html $300 for two 12inch Kicker CVRs) and for an amp http://www.sonicelectronix.com/item_13240_Pioneer+GM-D8400M.html ($210) with at least 4gauge wire
