sls, the amp will put ot 500W total, splitting the power between the two subs. Each sub will get 250W each.
If the 500W is at 4ohms, you will need to get 2 8ohm subs like some Kicker Comps or JL W0s. You can also use two dual 4ohm subs that take 200-250w RMS. I would suggest some RE 12s (REAUDIO.COM) maybe some Memphis 15-PR124D (all subs were 12s).
You could also get a better single 12 like a Eclipse Alum. DVC 2ohm or a Phoenix Gold Xenon 12D2 or Ti series. a Kicker CompVR, Alpine TypeR, Diamond CM312D2.
These are just basic ideas. I'm sure there much more you can work with.