I remember you saying you were going to buy that machine from another thread. It's the same as mine, woot.
I am running Xplane 8.6.4 and it is pretty realistic when you consider all the calculations it's doing but not as good looking visually as I would have hoped. Our machines with the Geforce 5200 won't allow much extras turned on and the game wants to do this fog thing in the distance, meaning it only wants to draw textures up to that distance of say 12 miles and the rest is a blue fog which looks odd. You would need to pick and choose which options you like more. For example I have turned off ground objects like buildings so that I could get x2 anti-alias and more visibility while staying above 30fps most of the time. It's also nice to take the X-30 and go into orbit or fly over mountains. Note that it does have small brief pauses but nothing too bad.
Still not bad for a 11 year old computer. I can't think of another flight sim that would work.