I just changed my lowly 633 MHz Celeron to an AMD 2600, 512Mb DDR Ram and an Nvidia 64 Mb card and let me tell you...what a difference. This simulator gets very impressive with full graphics. Clouds, scenery, special effects, water, etc, look truly beautiful. I am mostly maxed out in all the sliders and in the busiest airport I go down to about 12 fps, but it is still smooth enough. Usually I am running at 20 bps.
If you haven't upgraded yet, an you are an avid FS flyer, believe me it will be worth every penny. But keep in mind that what makes a difference in upgrading or changing systems for better frame rates and eye candy is: processor speed, lost of fast RAM (no less than 512 Mb) and a good video card (something in the >$200 price range Nvidia or Radeon with 8X AGP)