Yea you would have to be at around 4.2ghz to not cause a bottleneck with a 6870 crossfire.I have a 6870 crossfire so if you have any questions about it I can answer them for you.
The DoubleD card from XFX is a good choice but I would rather go with MSI or ASUS.The only thing that would sway me towards the XFX card is if they are offering unlimited warranty.But in the end they all offer pretty good temps.
PhysX is pretty much useless because only a handful of games support it.
As for the upgrade in general if you have the money I would suggest going for a 7850.It's a lot more "futureproof" than a 6870.
I'd also say go with the 6870. It's an excellent card for it's price. Also it comes in 2GB versions which is important if you are considering Crossfire. 2x 1GB does not make 2 GB in Crossfire/SLI. Same amount of textures need to be stored on each card, so if you think you will require higher res in the future (bigger / more screens), you should take that into consideration.
I just attempted my first overclock and that didn't go too well, so I don't know about getting to 4.2ghz. Will make a thread about that in the right section.
I'm not too fussed about unlimited warranties as I plan on upgrading my whole system within a year, so I'm just after something to be able to play games on in the meantime.
I can pick up the 6870 1GB for ~£120, the 6870 2GB for ~£160 or the 7850 for £200. Is the 7850 worth the extra £80 in my current system? I know it will be when I upgrade the whole thing, but that's not for a while yet.
I was just kidding.Their's no way you can get to 4.2ghz with a i5-760 without watercooling.Just get a simple aftermarket heatsink.It's pretty easy to get to 4ghz on the i5-760's with little effort but anything past that requires some really extreme methods.Something like this would work...