[mikegett]

It will depend on if you want one sub or two. If you want two subs then go with the lower end SW6123 with dual voice coil 4+4. You can place each subs voice coils in parallel to get 2ohms without bridging. The subs rms is 250 while the amps rms at 2ohms is 200 per channel.
If you want one sub then go with the SW7124DVC-T2. This is a much better sub and the 2+2 voice coil will allow you to run the sub in series briged at 4 ohms. The subs rating is 450 watts rms and 900 max. The amps rating is 400 watts rms bridged with 800 max.
The sw6123 will be 42 appiece while the sw7124 is 90. Here are the pros and cons. The sw6 will give you 2 subs for less, but are the lowest model made with a excursion of .78 one way. The sw712 is one of the best subs made (next to the titaniums) with a full aluminum cone and 1.4" (one way)excursions. These were the subs that started the spl crave in 97. They were the first sub to handle a thousand watts, had the largest excursion, and were the first to use aluminum cones. I owned a 8 inch and loved it. I jsut didn't have the power to push it at the time.