Not properly, it won't. If you enable the /3GB switch in XP, FSX might be able to access up to 3gb, but weather it will use it is a different matter. Also, you are then restricting the whole of the rest of the OS to only the 1 remaining Gb. And don't forget that your graphics card takes memory off this address block too, even if it has it's own onboard memory.
I fly through the valley of death, but I fear no evil,for I am at 80,000 feet and climbing. Let's powerdive and scare that poor soul in that Ultralight there.