This gets asked a lot, so here's a basic guide..

1. Get a better battery. No, its doesn't need to be a $500 Optima or Odyssey, just a nice Exide or Century or SuperCharge with 500+CCA & 120+ minutes reserve.. budget $120-180.

2. Upgrade the earths. Buy 4 gauge cable. Replace the battery -ve to chassis earth and engine block to chassis earth straps with the 4g. While your at it, run another 4g from the battery -ve to the firewall.

3. Upgrade the earth cables on your amps to 4g. The +ve should also be 4g.

4. Get an auto-electrician to check out the alternator and see how healthy it. You can also ask them to upgrade the alternator to battery charge wire to something bigger while you're at it.

5. Clean your battery terminals & clamps with steel wool or sand paper for a good clean connection.

In 99% of cases these simple steps will solve any problems you have with lights dimming, etc. Caps are worthless, spend the money on cable or battery upgrades.

And remember, I run 5000WRMS (380A current draw) into a pair of 15"s, 400WRMS into a 6" front stage (100A current draw) and a pair of Hella 100WATT spotlamps for a grand total of around 500A of current draw @ full song...
All off a single Odyssey battery & stock 60A alternator without the headlamps/spotlamps dimming at night.