I personally wouldn't do it as the difference wouldn't justify the cost. You'll definitely get a better performance, but it won't be anything mind blowing. But being "worth it" is a matter of opinion. I guess if you have the money, any upgrade can be "worth it". If not, I'd save up and upgrade when next generation of CPUs and video cards become available.
The answer to your question is dependent on a few factors.
First off, what does the rest of your system look like.
It very well could be that your current GPU, and thus a 4870, is bottlenecked bye your configuration.
If that is so, you would see no improvement in performance at all.
Secondly, what resolution do you game at.
If you are gaming at 1650x1080 or lower, you likely will not see to much of a boost in performance.
Lastly, what games are you playing and what makes you unhappy with your performance in them.
If you are already getting a steady 60+ FPS, it will make no sense to upgrade as most displays run at 60Hz.
Please answer the above questions and we will gladly help you make the correct decision.
And a single 4870 is simply never going to be faster than a 4850cf setup.
@OP Dont bother, the only worthwhile upgrade would be a 4870x2.
The extra memory is only going to be of benefit at very high resolutions, and at those resolutions, performance in recent games is till going to be choppy on a single4870 core....the 4870x2 or wait.
General rule of thumb, when you upgrade your video card, you want to make a 2 tier jump. Most of the time, upgrading to another card in the same tier, even upgrading to a card in the next tier is an expensive purchase for very little performance gain.
I can think of one exception, if you have a 3850 for instance, it would be worth the money to buy a 4850. But the biggest reason to make this move is because the 4850 has a really great price point for the performance you get.
It's ultimately up to you, but a 4870 is not going to be a eye popping difference over a 4850. Probably no difference at all on the vast majority of setups out there.
If you feel the need, yeah the X2 would be the way to go.