I had a standard 5870 that performed great but died a couple of weeks ago. I returned it and was very tempted to jump on that 5870 XXX at the Egg. Instead, I reinstalled my GTX 285 and am looking for a deal on another one. I think two 285s in SLI will outperform a single 5870 for less money. I say stick with your 4870x2. It's equal to or slightly better than the 5870. If you just "have to" upgrade your video card, go for that 5970 beast.
From someone who just went from two 4870 X2s to two 5870s in cf, there is no performance gain whatsoever in getting the newer card. You'll in fact lose FPS in some games.
Now, if you're planning on getting a couple more monitors for Eyefinity, then it's worth it.