The thing I've noticed with 5870 Crossfire X is if
I get occasional hard hitching in a title it's at 1920x1200 with
AA active. To go back to STALKER CoP; 1920x1200 all on/enabled save AA = smooth with good mins, 1400x900 all on/enabled with AA = smooth with good mins, 1920x1200 all on/enabled with AA = occasional hard hitching that ruins my mins.
It's possible the 470 does better in these situations due to its larger memory interface and higher amount of ram. It could also be a driver issue. I'm not sure.
The OP specifically mentions the 470 and 5850. Those are indeed his best choices. All my "derailing" was intended mostly to discuss min FPS between those two offerings in single and MGPU configurations. Often folks who want added performance may pick up an additional card for SLi/Crossfire later so considering that is relevant. Overclocked 5850 Crossfire performs remarkably similiar to 5870 Crossfire so I added my own experiences which reflect what a good deal of reviewers are reporting.
Yet again, IMO 34/55 is more desirable and reflective of a possible smooth gaming experience than 20/70. Games always feel more sluggish to me when FPS dips to 25 or lower for whatever the reason. I feel 30FPS mins or higher are desirable. Yes, that's all opinion and preference.
I'm not an "nVidia fan boy". My graphics card history is below. You may add triple HD 5870 Crossfire to the end.
I've got an EVGA 470 arriving Monday for computer 2, so I will be able to offer opinion based on direct experience with it soon.
In any case, I hope you enjoy whatever card you finally decide on purchasing