Ohm's you ask, -eh.
Ijust got my head around them myself
during my mission to install empeg a few months ago, so here 'goes.

Ohms measure the ammount of resistance on a given circuit.
For example purposes, let's say you have a 2 channel amp,
that does 100 watts on each channel at 4 ohms.

This means that if you connect one 4ohm speaker to each channel,
they will each get 100 watts of power. Simple so far.

Now let's say you put one 2ohm speaker on each channel, now the resistance is only 2 ohms,
so "theoretically" the speaker will draw 200 watts out of the amp.
This is contingent on wether or not there is enough voltage powering the amp and
wether or not there is a regulated power supply inside the amp.
Some amps are designed not to allow this theoretical "doubling of power"
some are built on the idea that you can, like Rockford Fosgate's Amps.

A real good way to understand this better is to consider your Sub woofers.
Let's say you have 1 Sub rated 500watts RMS at 4ohm.
And you have a 2 channel amp (250 watts x 2 chan @ 4 ohm) to run it.
First bridge the amp, giving the above mentioned amp (500 x 1 chan @ 4 ohm)

You do this by connectint the LEFT chanel positive to the positive lead on the sub
and connect the negative on the sub to the RIGHT channel's negative.
now your 250 x 2 chan @ 4 ohm amp puts out 500 x 1 @ 4 ohm, pretty simple
and perfect, since this is the exact specification of your sub.

Now let's say you want to add another sub.
Well first instinct is to say, well geez, I need another amp to run it, right ?
Wrong.

You can connect two 4 ohm subs together and they will present a 2 ohm load to the amp.
Do this by connecting Left Positive to positive on sub#1,
negative on sub#1 to positive of sub #2 and negative on sub#2 to RIGHT negative on your amp.
Now you are showing your amp a 2 ohm load which will
turn your 500 x 1 channel @ 4ohm into 1000 x 1 channel @ 2ohm.
Voilla ! free 500 watt amp.
Now bear in mind that you are taxing your amp a bit here,
and I wouldn't do this unless you have a really serious amp.
Do not buy a cheap amp and think you are gonna pull this off.
Rockford Fosgate Amps are great for this. I would recommend them for this.
And of course, as always, RTFM before you do anything.
If the manual is of no help, check manufacturer's website,
and also, your local installer can help you with specific wiring issues.

Also make sure you have some pretty serious power wire from the battery to the amps,
anything over 1000 watts total I would go right to 2 guage wire.
Run it from the battery down the battery side of the car,
(you want the shortest piece of wire possibe)
once it gets in the trunk,
you can put a distrobution block and take off whaterve guage wire your amp(s) require.
then be sure that you RCA wires run down the opposite side of the car.