Nothing coming out anytime soon; the single fastest card (two GPUs like your 5970) is the GTX 590 and HD 6990. Of those two choices the 590 is the best option primarily due to noise levels.
The GTX 580 is the fastest single GPU and is about as fast as your 5970. Assuming you didn't bottleneck your video card don't expect to see an increase in performance. If you're gaming at 1080p then the 5970 was more than enough. In fact half of the card (HD 5870) maxes out just about every game at 1080p.