Just to confirm, that FSX prefers memory bandwith over the CPU clock.
I lovered the multiplier and increased the FSB clock (8x390 4,4,4,12) and I get smooth 30 FPS (moderate to high settings). It looks like RAM speed is important for FSX.
I'm still wondering why I can get past 390MHz FSB (780 RAM) with my PC800 CORSAIR TWIN2X2048-6400C4 which is declared for 800MHz at 4,4,4,12 latency (no matter what latencies, CPU/RAM voltages).
At 390MHz FSB it runs torture tests perfectly at 44C CPU (planty of room), but on 400MHz it can't even boot. Like there is some blockage (???)
C2D E6600 @ 3125MH (8x390 3,125V aircooled)
ASUS 5PW DH (latest 1801 BIOS)
TWIN2X2048-6400C4 (4,4,4,12 780MHz 2,1V)
8800 GTX @ 1920 x 1080 on Sony TV
WD 150GB 10.000rpm