Stick with brand name amps. That will help you get something worthwhile. As for what Ross is talking about, peak power means absolutely nothing. Any salesperson that will only talk about peak power is not worth anything and should be dropped in a heartbeat. RMS power is based on continuous power, peak power is on a very short burst. Last time I checked, music lasts for minutes, not fractions of a second.
Another rule of thumb that I try to stick with is "balanced power". What do I mean by this, simple, the power (added up) running to the main speakers (the mids and tweets) should be the same power that you are running to your subs. Now, this all assumes that you are running speakers that all have the same sensitivity (measured in dB/W). If they are not the same (normally they are not), then you need to use a simple rule, 3X2. Or, for every 3dB less the subs are to the main speakers, you need to double the power. So, if your main speakers are say 89 dB/W efficient, and your subs are only 86dB/W efficient, you would need to double the power you are running to your subs in comparison to the main speakers. So, to put this into something more useable, if you are running say a 4x50 W (RMS) amp for your main speakers, then you would need to run 400 W (RMS) to your subs (4x50 = 200, then since the subs are 3dB less efficient, 200 x 2 = 400). With this being said, you can overbuy on subs really easy. Keep in mind that higher wattage subs are more stiff (have to be to handle the rapid movements of the cone), so, using say a 1000W sub with an amp that is only capable of outputting 200 W is a bad idea since the amp is not capable of pushing the sub adequately. Another one of my rules, the sub should be rated for no more than 2X that of the amp. This keeps the sub loose enough that you can get good sound out of the sub even at low volumes (relative to the strength of the stereo), yet, big enough that you are not going to destroy the sub the first time you crank it up. Keep in mind that clarity is important when it comes to subs. What destroys most subs is not overpowering them (they are actually capable of handling excessive power for quite a while), but having lots of distortion (aka, high frequencies). The distortion causes the cone to not move as a solid unit, but more as a rolling wave. This "whipping action" rips the edge of the cone. If faced with a decision of two amps, look for the one that has less distortion, even if it has a little less power. You will be happy you did that, so will your ears.
Another thing that I will get out of the way right now is WIRING IS OF THE UTMOST IMPORTANCE!!!!!!! HINT!!!!!! If you think you can skimp on the wiring, you are going to suffer for it. To give you an idea of what I consider adequate wiring, I use 6 gauge wiring for a 300 W RMS (total) system. By using larger gauge wiring, you are getting less voltage drop between the battery and the amp. If the amp is getting a higher voltage, it doesn't have to work as hard, therefore it will operate cooler (leading to long life as the electronics are not as hot, heat is what kills electronics) and since the amp isn't working as hard, it will also have less distortion. Now, there is a point where you are going to get less for going bigger. The rule I use with knowing if you have the proper gauge wiring is having less than a 0.5 VDC drop between the point where you are tapping off of for power and the input of the amp (this can be measured using extended leads on a multimeter) when the volume is at max. I'm sure you are thinking that heavy gauge wiring can't make that big of a difference, well, consider this, even having a very smaller resistance addition (0.01 ohms) with a stereo pulling 100 amps (roughly a 1000 W stereo), results in a drop of 1 VDC. So, you are making that wire dissipate 100 watts of heat (ever feel how hot a 100 W bulbs gets, thats the heat coming off of your wiring!!!!!)
If you need any help, let me know. I've been around the block a few times with stereo systems.
