W3rd! Ren is spot on. The best thing you can do for your power supply IMO is to upgrade you alternator cables to a high guage cable. I'm using 4 guage cables currently and my headlighs never dim no matter the volume. This will help much more than a capacitator and will cost less, it's just alot of work to run the wire, especially if it needs heat shielding along the way.
