250GB just for FS is fine. Send me a PM on the addons. If you're looking for scenery I'll let you know what to get.

I'm currently running FSX on 1gb memory and it's going just fine.

Just make sure you do the following.
Get the best GFX card you can get
1gb memory
best processor you can get.

Outside of that just go generic if you have to. 120gb is plenty for FSX if need be. Stick with WindowsXP too. After my experiments you don't want the other for this.