In the Mac Pro thread, we discuss and quibble about these things endlessly. There are no OpenCL apps at this point, only benchmarks. Thus far, in articles posted online and benchmarks posted by MR members, the 4870's performance has been abysmal - worse than every other card.
This doesn't make sense, given that the only cards that should outperform it (in a raw power sense) are the Nvidia Quadro and 285 cards.
Historically, ATI drivers have always made their cards much better performers for the OS and Pro apps, but recently there have been a lot of driver updates for various cards in system software updates, so it is not unreasonable to think that the card's performance may be different after an update or two. However, has recently been trending towards including Nvidia GPUs and their architecture may at this time be better for OpenCL applications what with the CUDA legacy.
Barefeats has not yet posted OpenCL tests involving the 4870 yet. They will.
Benchmarking is inherently artificial and not the same as speed tests involving an application in practical usage (which is what most reviewers, Barefeats included prefer to do). But since there are no OpenCL apps yet, we can only benchmark at this time.
All of these factors cause me to recommend that you wait before upgrading your card unless you have an extremely compelling reason to do so now. Why bother upgrading to take advantage of OpenCL if there are no apps that use it yet? It just doesn't make sense.
Right now, the best price/performance video card upgrade for your machine is to take a PC 4870 and flash it with a Mac EFI compatible ROM, if you were dead set on upgrading now. But for the reasons enumerated above, it is best to wait and see.
Hope that helps.