memory problems -what is best OS for FSX & Xplane9?

Dont know much about computers but have built my own system for flight simulation. System consists of following:

mobo: Asus P6T Deluxe
CPU: intel i7 920
RAM: 12gb ddr3 1600 corsair
vram: dual nvidia gtx 295 w/SLI
OS: Windows xp pro 32 bit

Been having a problem with xplane & fsx telling me i am using too much ram in settings. Sustem only recognizing 2gb ram. bios sees all 12gb. read some of info here & now know that my OS is the problem. So my question is "What is the best OS to use?" also, i will upgrade ram to 24gb when 4gb chips come out.

Thanks for help & suggestions.

I had lots of problem for flight simulation on windows vista and not able to over come of that but my bother manage to work it on windows xp and it working better without any problem on service pack 2

I would suggest a good graphics card which makes your system much better The thing is normal flight simulator Deluxe addition. Not the better version of it, gold addition. But may i alert you that when you install flight simulator it will slow down your computer and other game except your flight simulator