I just booted my i5-750 with only core 0 & 1 enabled.
Using my standard test flight and fsx.cfg (running REX/UTX) for 150 seconds I see the following frame rates:
4 cores - 11.92 fps
2 cores - 11.87 fps
That difference is within the standard error between any two tests. In other words, the 2 core frames per second is the same as the 4 core
My new mother board/memory/cpu cost me about $550
You can buy a new e8600 for $270 and use the same memory and mobo
The e8600 is 3.3GHz stock and can easily do 4 GHz on air cooling - I have no experience with that but everyone seems to report that and I know I went from the stock 2.4 GHz on the e6600 to 3.07 with little or no effort.
So - if money is of concern staying with your current mobo and getting a top of the line E series cpu would give very good results.
I needed a new motherboard - my THIRD nVidia 680i was failing with the same problem as the first two so it was time for me to switch motherboards and therefore upgrade everything.
ASUS P7P55D PRO / i5-750 OC'd to 3.4GHz / 4 GB Corsair DDR3 PC12800 / eVGA 8800GTS (640MB) / Vista 32 Home Premium/SP2