My PC is also from 2008 and I'm flying with X-Plane 10 with great add-ons (but without HDR). Start testing the demo:

http://www.x-plane.com/downloads/x-plane_10_demo/

 

What will be you GPU manufacturer in your new PC ? Having a good OpenGL driver is very important.

The latest nVidia driver seems to really give good OpenGL performance....

Why are people recommending the GTX680 ?  That's last years tech....

 

This year's tech is the GTX770 which has the same benchmarks as the 680, and is cheaper (at least last time I checked).

 

When I feel the urge, I'll be upgrading to a 4770k with a GTX770 4GB and 16GB ram.  But for now, I'm keeping that urge in check by flying to rural parts of the planet.