I have 2 500 gig HD's on my new puter (specs here)one of the HD's has Vista Ultimate 64bit, the other has XP Pro 64 bit.Which do you suppose I'd get better performance from with FSX?

The older the system, the better you'll be with XP. If you have a more modern system, along the lines of a Core2Duo or better, performance between Vista and XP start to even out as it relates to FSX.If your system is fairly new, I'd recommend sticking to Vista. Vista64 has far fewer compatibility issues when compared to XP64, and it will put you in a good position to move to Windows 7, which is far and away better than both XP and Vista. Good luck!-Greg