Your 4870x2 is still pretty powerful, about equivalent to a $200 card right now (6950, 560 ti). Cards in the $400-500 range would definitely be a noticeable upgrade, but not mind-blowing.
But enough of my bias; in that range your options for a single card are
gtx 680 - most powerful single GPU card, but
GTX 670 - best bang for the buck in the $4-500 range. this performs so close to the 680, it's not worth the extra $100 for most people.
HD 7970 - performs the same as a gtx 670, so not worth the money.
Since your motherboard supports crossfire, though, my vote is for
2x HD 7850 - massive performance for the $. It will max every game in existence @1920 without breaking a sweat. Performance is in between the 680 and the 690 (which is 2 underclocked 680's)