As in Microsoft Flight Sim X? That thing has CRAZY-high requirements and a quad-core (and good video card) are a must!
I didn't think that MS Flight Sim X could take advantage of 4 cores.
If you want only to change the cpu only... the best bang for the buck would not be a Q6600 but a X5450 or E5450.
Both of those chips are LGA771. He needs the LGA775 versions
Dual Core
=============
Core 2 Duo E8400 / Xeon E3110
Quad Core
=============
Core 2 Quad Q9450 / Xeon X3350
Core 2 Quad Q9550 / Xeon X3360
Core 2 Quad Q9560 / Xeon X3370
I wouldn't recommend a .65nm Kentsfield Q6600 that is sort of a step backwards coming from a .45nm E7200 Wolfdale. (other than the fact that is a quad core and not a dual core).
