It will also depend on what version of flight sim you want to use. FS2004 is very heavy to use by all accounts and you will get a very low frame rate. Although I ran CFS 2 on a similiar machine, to yours, albeit with a good video card and it was playable.
For your machine I would recommend
X-plane it is not as hungry as FS2004 (mostly due to the scenery not being as detailed) and many think it's flight characteristics are more "realistic".