What size amp should I get?

How does someone decide what size amp to buy?

Deciding factors such as...

Should highs and subs share the same amp?

How much power (or amps) are needed to run a speaker rated at certain amps?

etc. etc.

I'm new to 12volt.com and I searched the forums without any results for this. I figure this is a pretty important topic and lots of people would benefit from an answer.

From reverse order:  pre-amped subs already have an amp.  Running the whole system, a sub and 2 front speakers, on one amp does not diminish sound quality.  But trying to run too much on one amp will.  Basic rule is:  match the output of the amplifier to the RMS rating of the speaker. 

For example:  you want to power a pair of component speakers and a subwoofer.  You want to do it with one amp.  You see a nice 2 channel amp...stop...not enough channels.  You then see a 4 channel amp rated at 4 X 75 watts @ 4 ohms.  So then you select front speakers rated at 75 watts RMS each, and a subwoofer rated at 300 watts RMS.  Use this as an example and look at specs on a lot of amplifiers.  In this instance, two channels will run the fronts, and the other two channels bridged will run the sub.  And then you vary from that:  4 channel amp for 4 speakers and a mono amp for subs, 2 channel amp for 2 speakers and a 2 channel amp for subs,....etc.

You can't forget the $ and sound specifics.  Both are an integral part of equipment selection.  And there is no simple formula...that is why this forum exists!

To add to stevdart's good comments above, the only "formula" I can give you is more of a general rule.  FORGET about and IGNORE "peak" power ratings as they are meaningless.  Look at ONLY RMS rating of equipment at 12 volts.  This will give you the best indication of the true capability of the gear.  And never buy anything you have not heard.  Buying from specs alone is the quickest way to be disapointed.  You must like the sound and performance of your gear, not the ratings, the looks or how cheap it seems.

Thanks for the info stevdart and espaceman. It cleared up a lot.  Now for specifics.

I have a 2003 Nissan Frontier Crew Cab.  I plan on buying 6-3/4" for the front doors 5" for the rear doors and a four channel amp to power them.  As for bass I am definitely buy a 10" bazooka tube but I'm not sure whether to go with a built-in amp or provide my own amp and buy a tube that doesn't have one built-in.

What do you guys think?

I would like to make one correction to stevdart comment before I answer your question.  There is a way for you to use a 2 channel amp to power highs and a sub.  The only draw back is that you will have to use capacitators and high pass blockers or maybe a passive crossover.  Now on to your questions.

Forget the $ and the quest for a specific sound.  What I meant was is there a formula or rules as to figuring out how much power in the amp versus the power rating of the speakers/sub?  The only formula is, is to keep the RMS on the amp closely matched to the RMS of the subs. 

Does running highs and sub on same amp improve or diminish the power or quality?  There is no real sound quality issues by running sub and highs on the same amp. 

Do pre-amped speakers, such as Bazooka tubes, need to be connected to an amp?    Pre-amp speakers are already amplified and just need to be wired up

I would just buy the bazooka bass tube with the built-in amp, unless you are upgrading in the future to a better sub then you could use a 5 or 6 channel amp and use the 5th or the 5th and 6th channels bridged to power your bass tube. Then in the future you could change your subs without havng to buy an another amp.