Originally posted by: beemercer
Subwoofer (can be wired in 1 ohm):
http://www.sonicelectronix.com/item_3309.html
Amplifier (1 ohm stable, 600 Watts, built in crossover):
http://www.sonicelectronix.com/item_6521.html
Ported Enclosure (good for rap and hip-hop):
http://www.sonicelectronix.com/item_7500.html
Sealed Enclosure (good for rock and metal):
http://www.sonicelectronix.com/item_1880.html
Line out Converter (In lieu of a head unit with sub output):
http://www.sonicelectronix.com/item_1729.html
Wiring Kit (you can spend a lot more and get a better kit, but this gets the lob done):
http://www.sonicelectronix.com/item_52.html
I did a 10" setup because the 10" Kicker L5's are in stock, but if you have the cash, a 12" setup will be more powerful.
Also, as LS20 mentioned, you will probably want to upgrade the entire sound system. The subwoofer will over power the components and make everything muddy. As a result you upgrade the components (hopefully with their own separate amp), but now the head unit is the weakest link, or the vibration is a problem, etc. Eventually you find that you have upgraded every part of your vehicles sound system.