Triple monitors is the key thing here I think. If you were running just one screen, I would vote to sell your current card like you said, and get the 7950, hands down. With 3 screens however, all the reviews and tests I've read say that running xfire or SLI is THE way to go, the extra video RAM is crucial for running at those crazy high resolutions. Like Masher said, his 5770 xfire works fine for AH, so your 6870 being a faster card than the 5770 by a bit should work just as well once it has a twin installed in the slot next door.
Do a search regarding Xfire and single card/xfire solutions at TomsHW or overclockers/etc/etc type sites, every one I've read say that unless you have a purpose built single card solution for triple or more monitor gaming, such as the 7970 6gb Saphire Toxic, or the 690 GTX nvidia card, you NEED to be running dual cards in order to push the detail limits up at all at the 57xx by 12xx high resolutions you run with the 3 screens.
So, although the 7950 is a super card, VERY capable for single screen gaming, it won't be as capable as two 6870 according to the search I did. It'll be close in terms of single screen, the 7950 can be easily overclocked and tweaked to be withing a couple of percent of the lower end 7970's. Your BEST solution would be if you can afford it at least is to sell your current card and buy TWO 7870 or 7950's for your 3 monitor setup.