Look at the fuses that are either plugged into the amp, or specified by the instruction book. If you see a 400w amplifier with a 5-amp fuse, you should be suspicious. Remember what was said above, multiply size of the fuse by around 6 (12v at 50% efficiency), and that will give you a rough idea of what you are dealing with in terms of maximum possible RMS power.

It is a good idea to get an amp with a built-in crossovers, so that you don't have to spend extra money later on crossovers. If you are going to be using multiple speakers, make sure the amp is 2-ohm stable (or less). A bridgeable amplifier could come in handy in the future if you are planning to upgrade. Overheat, short-circuit, overload protections are good features that any good amplifier should have. Look for a low THD (total harmonic distortion) rating.

There are different amplifier designs: Class A, A-B, B and D

Class A amplifiers are the most sonically accurate. On the other hand, they have some drawbacks that make them a rare breed. Class A amplifiers use only one output transistor that is turned on all the time, giving out tremendous amounts of heat. Class A amplifiers are very inefficient (less than 25%). More heat means more heatsink area, so even though most class A amps have built-in cooling fans, they are big. Class A amplifiers are usually and expensive choice.

Class B amplifiers are the most common by far. They use two output transistors. One for the positive and one for the negative part of the cycle. Both signals are then combined. The problem with this design is that at the point when one transistor stops amplifying and the other one kicks in (zero volt line), there is always a small distortion on the signal, called crossover distortion. Good amplifier designs make this crossover distortion very minimal. Since each transistor is on only half of the time, then the amplifier does not get as hot as a class A, yielding to a smaller size and better efficiency (typically 50%).

Class A-B amplifiers are a combination of the two types described above. At lower volumes, the amplifier works in class A. At higher volumes, the amplifier switches to class B operation.

An increasingly popular kind is the class D amplifier (known as digital amplifier). These amplifiers are not really digital (there is no such thing), but operate similarly in the same manner as a digital-to-analog converter. The signal that comes in is sampled a high rates, and then reconstructed at higher power. This type of amplifiers produce almost no heat and are very small in size, but really expensive. Although there are full-range class D amplifiers available, most high-end manufacturers are designing amps for low frequency applications. These amps are capable of over 1000 Watts. Efficiency is much higher in class D amplifiers (~80%).

How much power do I need?
For mids and highs, anywhere from 30 to 50 watts (RMS) per channel would be a minimum. For subs you would need at least 80 - 150 watts (or more) per subwoofer. There should always be more total power going to the subwoofers than the rest of the speakers, since human ears are more sensitive to higher frequencies than lower. For example, if you have 4 x 50 watts going to all your mids and tweeters (total=200 W), then you should have at least 200 W or more going to your subs.

A lot of people wonder if too much amplifier power can burn up the speakers. What damages speakers most of the time is distortion, not power. If the speakers have the proper crossovers and are not distorting, then it is really hard to blow them. A bigger amp just gives you the opportunity to go to higher volumes without distortion. Get the biggest amplifiers you can afford and your car's electrical system can handle. More power means louder sound, but most importantly, cleaner sound.

Power?
There are different ways in which power is measured by amplifier manufacturers to make people think that their amps have more power than others. Laws of physics tell us that Power can be obtained by multiplying Current and Voltage. For example, if your amplifier gets 12 volts, and it draws 20 amps, then power would be 240 watts, right? Not exactly. In the real world, amplifiers waste 50% or more of the power in the form of heat. That leaves you with only 120 watts.