What's the rest of your rig and the purpose of it? Adding a 5970 might (most likely) be overkill. You're running near 5970 speeds already except what a 5970 is 2x 5870 GPU's downclocked to 5850 speeds and thrown on a single card.
In addition to that, the CFX scaling is slightly better on a 5970 than actual CFX between two cards.
If you are willing to drop the money on one, I would sell your two current cards and buy a 5970, odds are that will be more than enough. Plus assuming your mobo has room for a second 5970 you can have 4-way CFX at a later date which would just be complete and total overkill for just about anything.