hey and welcome to the board,
ok, if you are set on Rockford subs, that model would be fine, if you want you could look at one stop up from the the HX2's, they are fairly cheap on the net some where, I cant remember but someone on here knows, but the difference in voice coils (as simple as I can describe it) is that a single voice coil is the only connection that carries the apmlified signal, and a Dual VC is a sub that gives the user more options to wire up the sub, meaning if you have a single voice coil 4 ohm, it can only be wired up as a 4 ohm load to the amp, were as the dual voice coil that is dual 4 ohm can be wired up as 2 ohm or 8 ohm, there is series and parallel wiring, it is 2 different ways to run the speaker wire between the postive and negative termanals,
the difference in 4 ohms and 8 ohms is that the 4 ohm sub puts twice the ohm load on the amplifer, so the the amp has to work twice as hard to push it, with is not a bad thing in most cases, may amps are set up to run at lower ohm loads, I hope this helps some
good luck