Also if you fly Project Airbus, OOM is not your enemy even close. With NGX, T7 or similiar complex plane, chances to experience OOM are way higher, but in DX10 there is a bigger cushion as Bjoern stated.

Very similar upgrade here too. I was on Q9505 @ 3.5ghz, upgraded to i7 i3770k @ 4.5ghz on a Z77 board. Kept RAM 8gb @ 1333mhz and GPU GTX 560. Difference in DX9 mode is unbelievable. FPS is around 2.5x higher, and texture loading is much faster.

 

I'm currently in DX10 mode with the fixer, it's smooth of course, but FPS is much lower than in DX9 mode. Test situation results, DX9 - 190fps, DX10 - 80-90fps! I'm not really sure why FPS is over 50% lower in DX10, I must investigate that and maybe post a question in DX10 thread. Clean Win7 and FSX install. Tested DX10 without AA, not using inspector for now, only created profile in nvidia control panel.

 

I've seen this before as I use several different way to check FPS. It appears to be related to vsync in DX10 mode where sometimes my GPU monitor will report double the FPS that FSX is showing. But I have seen it go the other way also where FSX was reporting double of what EVGA precision was showing. It is always in full screen mode that this will happen. I believe it is because with  Windows 7 MS forced all the video card companies to have vsync on always for the desktop in their drivers so if you are in window mode your vsync should be on. This also has a tie in with windows aero being on or off ( I believe turning off aero also turns off the forced desktop vsync) 

I run in windowed mode with Aero on (via FSUIPC). With Aero on somehow vsync is active in windowed mode even without nvidia inspector and forcewindowedvsync=1 line in fsx.cfg. I tested the performance also without aero, no difference.