I have a Matrox G400 - Celeron 533 - 128mb Ram and the most recent driver updates (very important!) In FS98 my frame rates range from 60-90. In FS2k I get 18-60 depending on what I am running. Scenery is a killer sometimes and it as well that scenery add-ons should be removed if they seriously reduce FPS. Some people on the forums complain bitterly about frame rates but so long as you get around 18-30 you are OK - your eye only manages 25.
However, you do need DirectX7a and the latest patch from Microsoft - there are two, one for Standard Fs2k and one for PRO. You should be sure you get the right one. Patch 2b it is.