I believe it will probably do it at like 60 fps or something assuming that the rest of your pc parts are up to snuff. 60 is good though, so no worries.
Don't threadjack... but its up to you.. I'm a patient guy, because I waited like 6 years to upgrade from my crappy P4 rig to this... and Tech-Forums gave me 80% of my rig parts anyways . You may not be..
They're not even finished developing 4 series cards yet. I don't think the 5 series will be out for months. A crossfire of a 4850 and 4870 will last for a long time though, so you should be fine.