Here is an Infinity Ref. 12" Dual 4 Ohm sub. Build a nice sealed box for it, and get it proper amplification, such as
this.
Infinity is a decent all around company, so there is little doubt of product quality. I have heard good and bad things about the Profile amps, but the reviews listed seem to be in favor of it.
Check out the stickies at the top of the A/V forum page. You may want to focus on the
Ultimate Audio Sticky and perhaps zoning a little more in on the topic at hand,
Cam's listing of audio equipment which should answer most all questions of "Is this (insert product/brand here) any good?".
If you choose to go the DIY way, which I would suggest (for self installation, box building, etc...) then do your research before jumping into a system. You'll be much happier in the long run.