The rundown...
Ohm:electric resistance equal to the resistance of a circuit in which a potential difference of one volt produces a current of one amp.
Advice: Since it sounds like you already have your amp you want try to match your subs at the proper wattage/ohm rating.
ie: Say your amp is 400x2 @ 4ohm and 800x1 @ 2ohm. Lowering the ohm level will increase your power or watts.
Speakers on the other hand can be wired so they drop or grain ohm levels. It would take too much time to explain them in a simple way so here's a link for u to do some research on. http://www.termpro.com/articles/spkrz.html
Also, choosing correct wiring will be a benefit to your amplifiers.
ie: Think of 8gauge as a 2way road, 4g as a four way, 2g as an interstate and 0g as the autobahn. The larger the gauge the more traffic(power) you can pass through.
Look into buying a multimeter if u dont already have one, which reads ohm loads, i prefer fluke. This will make some wiring jobs less difficult.