700 KH/s is pretty good for a 7970, depending on how much you have it overclocked. I haven't been able to get mine past 650 personally, despite spending a lot of time tweaking settings.

As far as crossfire goes, everyone says to stay away from it, yes. I personally have had it on with CGminer doing SHA256 and scrypt and haven't noticed much difference either way. I leave it off though because there's not really anything I need to crossfire 7970s for that one 7970 can't do by its' self.