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.