xerxces :
You cannot CF those two cards and from my experience with CF, it is never a good idea. Always best to go with the single best GPU you can afford. I had 5 years of hell with two sets of AMD cards in CF and I will never do that again.
Actually, you can CF those cards, but you may need to flash a bios to one or the other. They use the same chip, but may not play nicely together if the software is different. You can always pick up an actual HD7970 for basically the same price if you are worried about compatibility.
To answer the question of value, it depends on what games you play and what you hope to accomplish. According to the anandtech bench, CFing these cards can almost double frame rates on some games. It comes at the cost of some irregularities in frame timing, however. Also, some games will not take advantage of CF.
I personally have two HD7970s in CF. They work very well for me and keep my framerates well over 60fps in the games I play. I have never had any trouble with them and the new drivers eliminated any issues I had with stuttering.