Sorry to disappoint you but even computers with those specs could not get decent double digit constant framerates when flying in high density areas, Especially when running on Ultra High Settings. All i know is i would like to hope vista and directx 10 will have a good effect when it comes to getting better framerates in FSX.
Maybe it is another Microsoft ploy to force you to buy the latest directx 10 hardware and of course the vista operating system in order to get this program to perform at it's optimum