I don't have a cap in my car and ive been told by a few people that they are usless. If you want to run more power id rather go with a bigger alternator.
For an amp you can go with the brutus by hifonics.
http://www.maxxsonics.com/hifonics/index2.html
You can also go with the stuff they have at elemental designs. They make high quality stuff at a great price.
http://www.edesignaudio.com/
JL audio is also another great option even though their stuff costs a lot
http://www.jlaudio.com
Also the subs made by adire audio are kick ass, even though their website isnt the greatest.
http://www.adireaudio.com/
Also you can go with Resonant Engineering for a sub...the xxx especially is amazing.
http://www.respl.com/