I was having happy times with my Q6600 2,4GHz that was overclocked to 2,75GHz, with 3 Gb of RAM and a 8800GTX with 768 Mb of RAM on it. I had a good visual quality and a stable performance as long as I was staying away from the major airports like JFK or Paris.
Nevertheless, I could make IFR flights to medium airports without any problems, and I had some WOAI packages installed. Not 100's of them of course, but some of the major internationnal companies in order to get AI aircrafts a little bit everywhere on the globe.
Don't get me wrong, I could land in major airports with a perfectly flyable sim, but the FPS would get around the 15-17 only in those case, which was not as fluid as the normal areas. In those normal areas, I was maintaining a stable 25 locked FPS limit. With unlimited FPS, it would vary between 25 and 40, depending on what was on the screen.
When limiting the visual settings in order to mach the level of detail of the FS9 world, My FPS were fluctuating between 40 and 70 FPS with a default aircraft over Seattle.
The Q6600 is a cheap quadcore computer. 2,4GHz is quite slow, but 2,75 was better, so if you can find somewhere a CPU that is faster, it would be a better choice. The 4 cores were very helpfull with photosceneries, too.