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.
Lol yeah thats what i figured as well. My rig is gonna be mining; nothing special and when i play games (BF3, C3, WoW, CSGO) I'm pretty sure 1 7970 can handle all of those maxed @ 1080p lol.
700 KH/s is good for a 7970? Awesome lol, didn't think it was that good.
What about MH/s (BTC)? What do you get? I get roughly 750 @ 1200/1600 Core/Clock
It should be around the same number except in MH/s.