CF would be a genuine upgrade 80% of the time. A crossfire setup is a nuisance, and a powerful single GPU setup is definitely preferred, but on the other hand, it is a very inexpensive way to get much more power.
Also note you may get similar raw fps with a CF setup vs a 7970, but that doesn't mean it will be the same experience. A 7970 will always work, and you will get more even/consistent frames, and less input lag, also, tessellation performance is pretty weak on 5xxx series, may be a consideration for games going forward .
If you want a beast system and you aren't too worried about cost, I would look into a different card upgrade. If you are at all concerned about cost/performance a CF could really make sense.