You would be better off asking about this at the AVSIM Hardware forum or the SimForums. Seems the real hardware gurus hang out there. I am not one of them, but I read all that stuff.

If you have not read Nick Needham's guides on setting up your operating system, loading FSX and tweaking FSX you really should, as HE is, by far, the ultimate guru on everything FSX ---- hardware AND software. His guides can be found here:

XP Guide -

http://www.simforums.com/Forums/topic29041.html

Windows 7 Guide -

http://www.simforums.com/forums/forum_posts.asp?TID=34141&PID=198187#198187

As FSX is so very much CPU bound I would guess that all the advanced tweaking, and high scenery settings, including LOD, require an overclocked cpu at 4.4 or above. That's along with the rest of a well balanced high end system.

I have a 5 year old system that was state of the art back then. It's a Q6600 quad core running at 3.6Ghz, 4 Gigs of ram and 3 WD 10000 Raptors etc and I run LOD no higher than 5.5. I also have Affinity Mask set at 14; Buffer Pools at 80g; and a couple other tweaks from Nick's guide. Am waiting for Flight and Ivy Bridge for a new rig.