I got the deluxe version of FSX about a month ago. I was unable to play it unless I had most settings on or near low. Even then I was getting pauses and low fps :-?.
I just got some new memory going from 512MB to 2GB and I was so pleased at the results in FSX. I now have mostly medium-high setting with aircraft set to ultra high. Weather and AI are rather low right now. The sim is running beautifully. I set the fps to 17 and it holds it just fine, though I haven't tried it over big cities or major airports yet. But where I was getting choppiness it is now very smooth and fluid, plus it looks much better

Before I got the memory I had tried higher settings, just to see what I was missing and it was unplayable (frame rates in the singles). Now it runs well and looks quite nice. I may even be able to turn up some more sliders now

I really didn't think I was going to be able to play it on this dated pc, but adding the extra ram made a huge difference!
My system is a nForce2, amd athlon 2100+ running at 200x11, 6800GT agp and the 2gb of Corsair TwinX cas2 memory.
So if anyone has a older system and are like me waiting for Vista and DX10 before your next build, try more ram in your current machine, you will probably be very happy.