Your amplifier is one of the most important aspects of your sound system. Buy the best you can, and get speakers/subs to match your Amp, and not the other way around.
When buying an Amp, look out for the RMS voltage, and not the maximum voltage.
Try to look for quality brands rather than cheaper brands claiming stupidly high output figures.
If your amplifier is of good quality, the sound will be too. Try to look for brands such as Vibe, Kenwood, Infinity, Alpine, Pioneer.
Subs and speakers?
If you’ve decided to amplify you subs and speakers it would make sense to buy two separate amps.
The reasons for this are that when a sub is drawing a lot of current, the amplifier will also send this to the speakers, and fry them.
Also if you over load an amplifier it can catch on fire, I've seen it happen and it’s not good.
Different types of amps will be better suited to different jobs.
How to set up your amplifer/s (click me)
Subs:
Here you are looking for a mono-block amp. This means that it only has one output, but this can be bridged to more than one sub, depending on the output capability of the amp. For example, the Vibe monobox, which is a very powerful amp, is stable down to one ohm. This means four subs of impedance 4 ohms can be used at one time.
If you are just running one sub then there is not much need to spend a great amount of money, and a small amp will be suitable.
A normal 2/4 channel amplifier will be perfectly adequate; the reasoning for using a mono-block amp is because they are specifically design for bass speakers at low frequencies.
Speakers:
Here you will either need a two-channel amp, or a 4-channel amp, depending on how many speakers you want to install.
This will not need to be as powerful as the sub amp, and about 100-watt RMS per channel is plenty to be heard outside of your car. Remember quality is better than quantity. And with a power output like that, speakers to match will start to get a bit more expensive. For the normal set up, a 300-watt amp with a 50-watt RMS will be fine. When the speakers are connected to an amp they will produce clearer sound even at high volume levels, and it is a lot harder to make them distort.
How much power will this draw from my battery?
Running more than one amp on a big system may draw more current than your alternator can charge. This happens a lot on smaller cars such as 1.1 litres.
If this is the case it may be wise to upgrade your alternator to something off of a higher specification car with air conditioning, electrically heated seats and so-on. The more specs the car has, the bigger the alternator will be.
It is advisable not to keep the system on for long without running the engine as your car may become hard to start after the battery has been run down.
For extremely big systems extra batteries can be installed to keep a constant amount of power supplied.
Most amps will be fused at around 20 amps, and some bigger amps can have up to four 40 amp fuses, meaning they draw 160 amps. It is important that adequate wiring is installed.
Amp troubles?
How loud is a Watt?
There is not a set answer to this question as it depends on your set up efficiency, and also the scale of decibels against wattage is logarithmic. This means that double the power will not make it double as loud. This is the reason why amplifiers become so powerful and expensive when building a huge sound system.
Not real figures, just a representation, power(x axis) decibels (y axis)