Hi Jasonp, Firstly, You didn't give your full system specs, neither did you mention the games you wanna play.
IMO, if you're looking at SCII, WOW etc. there's no real need to drop the $60 more (especially at 1680 X 1080). Your PSU permitting, you can get a second 6850 once you do decide to go for a full HD display.
However if Xfire is not your thing or your PSU wont comply, might as well drop the extra moolah for the 6870 especially if you're looking at Crysis, Metro etc.
Disclosure: I must admit I've not even seen a 6xxx series card with my own two eyes as yet. So my suggestions are purely based on reviews from tech sites on the net