If you're playing those game you don't need more than a 6850 lol. Try Crysis and Metro 2033 and The Witcher 2 - put that card through it's paces!
Anyway I doubt there's need to worry about CF right now. That card will play just fine for a long time. If you want max settings in all games at 1080p or higher you'll need to go CF... Anyway 7850s should easily be available at retail for 2 years and longer if you don't mind buying used. I don't see any need to rush out and spend cash unless you feel the need for more performance.
Well the theory applies to all CPUs and GPUs, but as for how much it increases I don't really know. I'm sure other overclocking reviews of other CPUs test the power usage, but this is just one I saw recently.