Both of the machines mensionned on this page are to be ok imo.
Flight Simulators have been easy to hard to the hardware. What I beleave is what ever macheene will run the windows Vista in the minimum system requirements will be able to fly good in the FSX
Look at Flight Simulator 9, I have a P3 550mhz & a GF2 pro 32meg and ran the game nicely, just can't pull details. Before the P3 there was a Celeron 500mhz. It improved alot and I dont think the 66mhz jump to 100mhz made a real difference. The architecture of the cpu really made the difference. At the time FS9 was optimized for 512k of L2 cache for the Pentium 4 Northwood. So my little p3 had the space for the total code .
Get the cpu with the most cache it will help alot, especially for 64bit applications.