You have a great video card there, and enough RAM - the CPU is going to be the bottleneck. MWO is abnormally CPU limited, and most of the folks with good performance are running quad-cores. Still, I play from time to time on my HTPC with an older AMD dual-core (Athlon II, so lower-end than your Phenom) and it does okayish.
If I had to guess, you will probably average around 30fps with that setup no matter what graphics settings you use. I would start with the highest settings in-game and see how that does; if you can't keep up 30fps+ reliably then I would scale down to see if it helps. Because of your video card, though, I don't think the graphics quality settings will help a ton if you aren't able to reach comfortable FPS.
The good news, though, is that you could probably move to a quad-core Phenom pretty easily if you want. Post back about how well it plays, and if you want to look at upgrading let us know and we can link you to good places to get parts