Regarding amps:
I will do my best to keep this as basic as possible.
2 or 4 ohms is referring to the resistance that the sub presents. As a general rule for most amps, as the resistance value decreases, the wattage increases. For example, if you look at the specs for an amp it will list something like 500w @ 4ohms and 700w @ 2ohms.
With class 'D' amps you can sometimes get amazing increases in power by lowering the resistance.
When buying a sub/amp combo, you need to think about how many channels you need, how many subs you are buying, and how you plan on wiring it. When wiring, most people "Bridge" their amps whenever possible to get the most out of it. What this is is simply connecting 2 subs to one channel to get more power from the amp. For example, 2 4-ohm subs run in parallel (bridged) will produce a 2-ohm load and should run at a higher wattage. Make sense?
The Kicker L7 subs are available in a variety of configurations. You may also be wondering what DVC means. DVC means Dual Voice Coil and is sort of like having 2 subs in one. If you bought the 4-ohm DVC unit, you could bridge it to run at 2-ohms without having to buy a second sub. This can be especially good in situations where space is a factor.
If you plan on bridging, you will need to find an amp that is stable at the restistance (ohms) that you are going to use. Also, a lot of people try to "match" the rated power handling capability of their subs with the rated power outpu of their amps. This is actually not usually the best thing to do. Most of the time when people damage their systems, it is because the overworked their amp and this can actually damage both the amp AND the speaker. The L7's are rated at 600 watt power handling, but I would probably buy AT LEAST an 800 watt rated amp to power this.
That's it for now, but let me know if you need clarification, suggestions, or help with anything.