you either have a dual 4 ohm or a dual 2 ohm bud. and the RMS wattage on the new 15" R's is 750, but they can take more than that.
Edit: from the sounds of it you have a dual 4 ohm? you'll want a sub running at 2 ohms to power that, and i would probably want to push around 750-1000rms @ 2 ohms and that thing will bump