The 5870 is actually a tiny bit slower than your 4870x2 for the most part, a 5970 would be a significant upgrade but it would also be an expensive one. Are there games that you cant run as well as you wish you could? If not then its not worth changing cards just for DX11.
Well i had a 4870X2 before and when i sold it(because of problems),i got a 5970 as a replacement,because 1 5870 performs pretty much like a 4870X2.
Here are my thoughts
5970 is about 30-40% faster, also runs cooler,quieter, and has less power consumption; furthermore, it packs newer techs such as DX11,eyefinity too.
Aaron- I wouldn't upgrade. Not for a while at least. You have plenty enough horsepower to play the games you want. Crysis isn't a fair evaluation of that fact.
I have a 5970, and its overkill for most games that I play, but I wanted to have something that I would use for a long time, and it met that expectation. Thats what your X2 will do for you. I'd wait until the next gen of cards (IMO).