I'm going through a similar trial-and-error configuration mission myself.
I have 2 5870 cards in CrossFire (Sapphire, 1 is the 2GB Eyefinity version, the other is a 1GB Game Edition).
When playing Crysis (or Crysis 2) I get absolutely awful performance from the cards. On a single card, the FPS is low, averaging 15, because I'm at such a high resolution (2650x1600). I was assuming that CrossFire would give me close to double performance, but no luck. In reality, the game is unplayable, with incredibly sluggish peformance, almost as if I'm playing the game on a 486 system or something. I have an i7-960 and 12GB of RAM @ 1600...so I know that isn't the bottleneck.
Sometimes, I wish the really high end computer components didn't have tons of configuration options, but rather just simply ran with the absolute best performance simply because you pressed the little red button that says "fix it"!
Anyone know of a site that tracks all the optimal 3D Application Settings in CCC for various games?
Thanks, esp. to Tex-Twil for letting me jump on the post.