I would imagine the 4870 CF will be slightly quicker but it will probably end up being more expensive (around $600) whereas (according to rumors) the 4870x2 will only cost $450.
EDIT: Yes, the 4870 will use GDDR5 and basically, you can clock it too higher speeds, it runs cooler at a set speed and it is generally more efficient. However, it is more expensive to manufacture and there is a somehwat short supply at the moment.
Judging from what's been said about the 4870X2, it actually might be faster than the 4870CF, granted the cores are not slightly underclocked. The reason is because the computer apparently recognizes the 4870X2 as a single card, so it should scale almost perfectly and you won't have to worry about CF scaling issues. How well this new technology works and if it delivers as promised will answer your question.
That said, I'm sure you'd do fine with 2 4870s in CF.
im going to stick my neck out and say the 4870 X2 will be faster than 2x280sli.
hope i don't regret that
Well the 4870 X2 may be fast, but it does not beat 2x280 in sli...
If the 4870x2 is faster than single 280 (we only hope so, but it definitely seems to be so if the specks hold...) and the scaling of 280 is normal 60 to 80% the 280 will beat the 4870x2 hands down, but how much the difference is? It can be so that the 2x280 in sli is 50 to 80% faster than 4870x2. If the difference is 50% the 280 has lost the battle in most cases. If the difference is near 80% the 280 in sli will have a boothold in extreme systems.
But if you mean that you would use two 4870x2 in cf, well it all is about scaling... If the scaling is really good it can be faster, but normally 4 GPU system has had worse scaling than two GPU's systems.