Keep my card or get new?

Here is the benchmarks for 4870x2:
http://www.anandtech.com/bench/Product/174?vs=158

Here is the benchmarks for GTX 570:
http://www.anandtech.com/bench/Product/513?vs=518

Unfortunately they don't put the 4870 crossfire configuration into the 2012 GPU benchmark.

From Tom's Hardware benchmarks:
http://www.tomshardware.com/charts/2011-gaming-graphics...

Overall, it's just a stronger card.

If you are looking for noticeable performance, you may not get much. Tom's have the 2x4870 & 570 at the same level in the hierarchy.

I quote from: http://www.tomshardware.com/reviews/gaming-graphics-car...
You can use this hierarchy to compare the pricing between two cards, to see which one is a better deal, and also to determine if an upgrade is worthwhile. I don’t recommend upgrading your graphics card unless the replacement card is at least three tiers higher. Otherwise, the upgrade is somewhat parallel and you may not notice a worthwhile difference in performance.