Do you ever plan on going bigger in amp power? If not....you could use 500w rms subs. If you ever plan on getting another 1500, you can strap for almost 3400w, so you would want 1000 rms subs.
If you want room to grow and want serious subs on the cheap, I do have available, 3 DC lvl 4 12's with lvl 5 softparts. They would be good on 500 watts, great on 1000 watts and amazing on 1500 watts. They are dual 1 ohm.
On the other hand, if you never plan on upgrading amps, you could so with a sa-12....nice sub at a nice price.