The efficiency of HYDRA is slightly less efficient than standard Xfire and standard SLi for cards of the same type, and less efficient for mix-and-match ATI and Nvidia cards. The technology at the moment is very new.
For Hydra, Sli, or Xfire, combining a high level card with a low level card is NOT recommended. Unless you're just using the low level card for physX or something, in that case you don't even need Hydra.
If you don't have a hydra board, just get a x58 or x55 board, those can do either Xfire or SLi. If you have a hydra board, just get the same card of the same type, or get an Nvidia card and pair it with an ATI card of similar performance. (ie. GTX260 with a 4870)