I truly believe vista is 'less stable' for handling FS9 and FSX. WinXP seemed to hanle them both quite well. With WinXP, I never had so many CTD's and issues with files disappearing. Also, vista seems to increase the 'stutter', (wierd jumps and things in FSX). vista seems to take 'moments' booting something up. It will appear to be frozen for 1 to 3 seconds. By that time, I have reclicked a program 2 more times, only to find it boots up 3 of the same programs. (Seconds counted as 'one one-thousand, two one-thousand, three one-thousand). FS Bootup time seems slower as well, waiting for a screen to popup, etc.
By the way. You cant run DX10 with WinXP, as you probably know. DX10 is designed for vista only. Thus DX9 is the last version of Direct X for WinXP. (I am sure you already knew that. Just wanted to add that for others that may not know).
Bill