Hmm seems ur gpu is the bottle neck, if i run only one gtx 480 my fps gets halved to the 30's
Your post caught my attention since I'm having the same problem as the OP.
Hardware: Intel qx9650 with a safe overclock, a single EVGA GTX 480SC, 8 gigs of Kingston HyperXRAM. 1920X1200 everything Ultra and maxed out all the way.
I can play the game for a while with nice performance indoors and outdoors but then there will come a point eventually where my performance outdoors begins to decline.
Is that a memory leak or just the nature of the beast? Am I being too greedy with my settings on a single GTX 480? What settings are you using?
What's interesting is: My sister has a comparable rig to mine except she's using a GTX 570 and she has not had this problem. She's ultra, 1920X1200 and it's not happening to her.
Be interested to see what you all think.