Best upgrade for performance-processor, video card or memory?

What can I upgrade to increase frame-rate for MS FSX (flight simulator)? When running FSX, I can’t get the frame-rate over 20 with all the sliders set past 75%. I’m running the resolution at a maximum of 2560 X 1600. My goal is to do whatever I can to increase frame-rate. The CPU temperature runs between 68 and 90 degrees during game play.
I’ve considered adding memory, changing video cards and/or changing the processor to i7. I’ve also considered a different cooling solution, but don’t know where to start. I would appreciate any suggestions.

Existing system:
Asus Maximus Extreme motherboard.
4 x 1 GB (4 GB) OCZ Titanium 240-pin DDR3 SDRAM1600 Dual Channel memory.
An Intel Core 2 Q6700 Kentsfield 2.66 GHz 2 x 4 MB L2 Cache LGA 775 Processor.
Two Diamond Viper ATI Radeon HD 2900 XT PCIE 1GB GDDR4 Video Cards running in Crossfire mode.
Asus Arctic Square CPU Cooler and an auxiliary fan.