I had xp + Intel E5200@2.5ghz + 4GB + GTS250 gpu, I could play the game at 1280x800 with everything low expect model details and decals at highest. Some stutters could be seen, just like everyone has.
Then I upgraded my gpu to Asus 6970 with 2GB mem, I could see very very minimal increase in FPS. Then I updated to windows 7 and immediately I could play at full resolution + high details (with the april 15 patch though).
So I would suggest you to update to windows 7 if you want to play this game
I'm a happy pilot now.