I think the Xt-2000D is rated at about 750 watts RMS (the 4000D is rated at 1500) and I would expect it to do right around there. I haven't used any of these new US Amps and the Xterminators are their lower line but from what I've heard still a solid amp and you should get your money's worth. I think I saw new 2000D's for about $200 on Ebay not too long ago.
Whether it's enough power depends on what subs you're using and what you want out of the system. If you could fill in those blanks a little bit for us it would help everyone make an informed recommendation. With $500 you can get alot more power if you're willing to buy online.