These questions are always difficult to answer as different people want different things from flight sims. The short answer is that your system specs would run FSX but you would need to run FSX at low settings to get playable frame rates. This means turning down graphic settings and reducing amount of AI traffic. For some people this is not acceptable and I can see why as you start to miss out on many of the features FSX introduced over FS2004. It's also worth bearing in mind that many of the available aircraft add-ons are hugely complex, replicating entire aircraft systems and would bring your system to its knees. The same goes for scenery add-ons. Can I suggest a couple of things?
1. There is a FSX demo available on the Microsoft website. Install this and see how your system copes. Bear in mind though that the demo has less complex aircraft and scenery is quite sparse.
2. There's an active forum at flightsim.com. If you post their someone may have the same specs and be able to give further insight.
3. You could try FS2004 which requires less processing power and will run quite nicely on your system. You can pick it up quite cheaply now. If you get "bitten by the fs bug" you can perhaps look to upgrade to a newer pc capable of running FSX in all it's glory.
Hope this helps?