The cards need to be the same model to work in CrossFire. You can't mix a 6870 with a 6670 or any other model. It has to be the same model. With regard to the 6670 - I do not recommend trying to Crossfire them (if it's even possible - on that I don't know). The performance of 2 x 6670s would probably not even equal a single 6870. If you want to stay AMD then get a 6870 and consider Crossfiring that card if you aren't happy with the performance. A 6870 is a big upgrade over a 6670. Make sure your PSU is up to it.
my psu is cooler master eXtreme Power Plus 600W. will it be ok if i install HD 6870.
what will happen if i install 6870 along with 6670. which one will work. is there will be any problem in the board.