Op can even use a HD6850 to CF with his HD6870.
BTW, Op has i7 950 cpu and he is probably going to get a tx 750 v2
Try to cross fire with same rated cards. If you join an 6870 to a 6850, the former will run at the speed of the latter. Else do not.
Yap, u r right, rather i should mention 'any 68xx gpu' .. But in cfx gpu will work in lower clockspeed (which gpu has lower clock). He can oc to match gpu clock though.
Regarding Crossfire, HD 6870 and HD 6850 can be crossfired and they will work as per their own frequency. Unlike SLI, in Crossfire both the cards can work at different Core and memory clock speed. So HD 6870 and HD 6850 will work at their own frequencies, 6870 won't be downclocked to HD 6850's clock speed.