Yes, you can CF ANY brand as long as it's on the same type of card, in this case HD5870 (even with different clock).
Your PSU is more than enough to run dual HD5870, so don't worry about that, dual HD5870 will crush almost everything easily...
^+1 to wa1. Also, running 2 differently clocked 5870s isn't an issue (or vendors) as the system will negotiate to the lowest common denominator (speed) when the 2 GPUs are synched for CF use. Additionally, if one card has 1GB or memory and the other has 2GB, the second card will only use 1GB while in CF mode.
Regardless, 2 5870s in CF are quite impressive performance-wise. They will also generate a lot of heat (and use a lot of power). Your PSU looks good, but also make sure your airflow in your case is good as well.