For the price, you really cant beat a pair of Rockford Fosgates. I've got 2 10 inch XLC's in my hatchback and they really bump. The regular Fosgates handle 100w RMS, but if you need to handle more than that, the XLC's will handle 200w RMS. The 10's dont hit the really low shit at all, so my system doesn't rattle windows from 2 streets away, but it's definately loud enough to do serious hearing damage. 10 inchers have the optimum response time, so any kind of music with fast bass hits (rock, some techno, etc) sounds absolutely great.
Go with 12s if you want the gut shaking lows though. You wont get great fast bass, but it's all a trade-off.
And DEFINATELY get a sealed box. Different types of boxes will be louder, but will sound like shit comparatively.
With respect to the amp, this is what makes your system. Dont get good subs and a crappy amp. Spend the dough on a good amp and get lesser subs if you have too.
Here is my personal opinion about the amps Ive heard and played w/ in mine or my friends cars.. (These are only amps i've used/heard so no flames if i left something out :] )
Bad: Pyramid, Optimus, Jensen, Kenwood, Profile, Sony, Rockwood, Kengate, Krunch
Good: MTX, Infinity, Rockford Fosgate, Kenwood eXcelon, McIntosh
Here's a link to a deal on a nice MTX. Check the web for better deals though of course
http://www.crutchfield.com/cgi-bin/S-q0vebPK1koV/ProdView.asp?s=0&c=3&g=110&I=236TH2150X&o=&a=0
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"Every program has at least one bug and can be shortened by at least one instruction -- from which, by induction, one can deduce that every program can be reduced to one instruction which doesn't work."