it cannot be made 'properly' multi-processor aware
If it is CPU-bound and only weakly multi-threaded, that's actually an argument in favor of the i5. HT is only going to be of benefit in situations where you have more than four threads and each thread can't fully saturate the core it's on. So in this case (as with most any case for gaming), you'd likely be better off shifting the price premium for the i7 towards a higher clocked i5 instead.
And triple channel memory isn't really of any significant benefit on quads (part of why it hasn't been available on anything but hexacores since that first generation).
Granted all this is moot in our case because the i5 didn't exist when you built that machine, but it's useful information for anyone looking to build/upgrade now.
This. Unless the FSX is not superbly multithreaded, i am pretty sure i5 3570 would smoke first-gen i7s fairly easily, possibly even the sixcore ones, because of higher clocks and higher IPC given the architectural improvements between the Nehalem and Ivy Bridge,,,
