I'm reinstalling FSX and upgrading some of my components. Is there any advantage to going with 4GB (2x2GB)Crucial Ballistix DDR2 800 RAM over (4x1GB) Corsair XMS2 DDR2 667?? Thanks

For CPU O/C'n, the 800 will allow a 400Mhz Front side buss where the 666 will only let you go to 333. However unless you plan to O/C the CPU it won't matter a bit, either way.

My MOBO is an Intel 975XBX with limited OC options. However, I plan to OC the FSB.

The 975 will go to 400Mhz. The Mobo needs both FSB speed and CPU core voltage adjustments. If your CPU will go there, you're there. Get the DDR2800 stuff (2x2). It's so cheap, there's really no reason to go with the 666. Then you'll have it.Of course you'll need a good CPU air cooler too. here: http://www.directron.com/u120e.html