SC2 is not gonna have near the requirements of Crysis in my opinion. FPS's have much higher requirements in comparison with RTS's. The card you have now would probably be fine for near maxed if not maxed. A 4870X2 would be overkill I think.
OP is rather clueless, 5870 is quite obviously designed for the PCI-E 2.0 x16 socket.
On a less taunting note, theres no way of knowing how the 5870 (assuming of course thats what they call it, I thought i7 was going to be Core3) will perform. Also equaly unknown are system requirements for SC2. Stop dreaming about the future and look at the present if your going to purchase hardware.
While a single 4870 is usually faster than than two 3870s (3870X2) I don't think that the 5870 will be as dramatic a jump over the 4870, but we'll have to wait and see. After all the 4870X2 only seems to average a little over 1.5 times the power of a single 4870. Even with a smaller manufacturing process its going to be hard to create a single GPU that obtains that kind of performance while maintaining sensible yields and power requirements as AMD currently tries to do.
The rumored 5000 series will be DX11 parts and are still months away. You should also keep in mind that a refresh of the 4800 series cards will be launched soon based on a 40nm process I believe. That means you will probably see a 4970X2 or something. I say that if your 3870X2 is fine for now you should wait till it no longer suites your needs to upgrade. If you want to upgrade soon though then wait a bit as the 4870X2 is dropping in price and may soon hopefully get down to the price range we saw the 9800GX2 at once the 4970X2 (or whatever they choose to call it) launches.
Unless you a fortune teller we all blindly guessing what the next generation cards arelike. Personally the hd5xxx wont be a huge jump because ATI sorted out most their problems when jumping to HD4XXX from the HD3XXX series.
I would go for a 5870 if ati works on the drivers because theoretically ATI's cards are better, but dew to good nvidia drivers they perform better or?
And theoretically, communism is the best social system that can be ... but humans can always find a way to make a crappy implementation of a good idea.
ATI is, perhaps, more advanced in miniaturization process (Nvidia is reaching 55nm and ATI is going 40nm in H1 2009 I think), but smaller isn't "better"; nothing would prevent them from making a highly power efficient crappy GPU .
ATI vs NVidia, AMD vs Intel, in the end as long as they keep trading blows the consumer is probably the winner.
In theory you can't compare the cards, really, because architectures are so different. If you're commenting on that ATI uses 800 shaders vs. 128(?) that Nvidia does, there's a big difference in their shaders.
As far as drivers go, I like ATI. They release monthly, and you don't have to rely on beta drivers like Nvidia owners usually do. (Though Nvidia often pulls a beta driver out for new, demanding games).
Funny hwo everyone complains about ATI driver issues whenever they come out. Sure, NVIDIA doesn't have tons of driver releases, but the betas are generally rock solid stable. I view a lot of catalyst releases the same way I view NVIDIA Beta releases.