What improvements can I expect from upgrading to the i7 and Nvidia 295 as compared to my system that was built in 2007.  My current setup includes a qx6850 oc to 3.45 ghz,  xp64, evga 780i, 8 gb OCZ pc6400, 2 XFX 8800GTX sli, and a 30" LCD at 2560/1600.  My software includes FSX sp2, FSGenesis, GEX, FEX, UTX and Nhancer tweaked as recommended except changing AA to 16S. FSX settings for scenery are set at the set to the max except water mid 2x, mesh 10 meter and no ground shadows.  Weather is set as Nick suggested and all traffic is set to 20 %. More traffic is annoying to me. I have gone back and forth on fps settings finally deciding on 30 fps.  This system runs at 15 to 20 fps in dense scenery areas at night and pegs at 30 fps at all other times.  

Forget the cost difference,  what will an upgrade achieve?

i7 achieves more smoothness than anything. you may be getting 15-20fps in dense areas, but if it's smooth then an upgrade would be unnecessary at this point. remember, you can throw the sliders down while flying above areas that are too dense... we i7 users still have to do that as well, especially around areas with so many trees you can't even see the ground textures. i7 is the best so far, but it's not perfect