Moe,
I've never run FsX on XP so I've never been able to compare. I've locked FsX to 20fps and I rarely see it drop below 20. Frame rates in Dx10 are a little better for me and I don't get those little "jumps" in Dx10 that I got in Dx9c. However after the thread about "Deleting the page file" and deleting mine I don't get those jumps in Dx9c anymore.
You will know, I'm sure, about disabling UAC and Indexing in Vista both make quite a difference! I can get 50 or 60 fps in sparse areas but 20 does me fine.
I noticed the otherday that I have far too many processes running (60+) so I'm trying to get to grips with "Alacrity".