Just an FYI , your subs can never have too much power handling but your amps can give too much power on lower rated subs.
You won't be disappointed with the RE's but if you plan on getting amps to match you'll need to upgrade if you haven't already.
If your total system power is 1200 watts RMS or more, you will need to:
1. Upgrade the "Big 3" - that is to replace (or add to) the power wire between the battery and alternator, the ground from the battery to the chassis and the ground strap from the engine/tranny to the chassis.
2. Upgrade to a yellow top battery
3. Upgrade the alternator to a high output alternator
Failure to do this will result in the early death of the alternator as it will eventually fail.
Here is a guide that will calculate what size alternator and power wires you need http://www.box.net/shared/uchv4dbk88
You will need Microsoft Excel. If you don't have Excel, use this link http://www.box.net/shared/nkkw1dhk4g
A second battery is never a good idea as it's an added strain on the alternator to charge two batteries AND supply power for the car. The ONLY time a second battery is useful is if the audio system is to be used with the car off.
A capacitor is only good so far as the audio system isn't trying to pull too much from the electrical system. A cap doesn't provide more power, it's designed to 'stiffen' the voltage to the amp, nothing else. If the current isn't there, a cap won't help.
The alternator must have at least 20% more amperage power than the entire vehicle and sound system combined for a cap to be of benefit which is ironically the same requirements for an amp to be efficient.
The correct way to set the gain:
http://www.box.net/shared/uchv4dbk88
You'll need a multi-meter (AC voltmeter, $15 from Radio$hack), Microsoft Excel and a way to burn an audio CD from an MP3.
If you don't have Excel, use this link http://www.box.net/shared/nkkw1dhk4g
See my site for more info http://spkrbox1.spaces.live.com