Well to be honest that's not an ideal system for FS2_Open, since the game is only able to utilize a single core on the CPU, so you're limited to the power of one core at 2.8 GHz. Similarly, the 9400 isn't exactly a powerhouse GPU, but you should be well able to play with all the enhancements nevertheless, especially if you get a recent 3.6.11 nightly build which have a bunch of optimizations.
Of course, I have Athlon 64 X2 6000+ at 3100 MHz, maximum amount of memory useable by Win32, and a GeForce 8800GT 512MB, and I'm able to get very much passable frame rates for almost all conditions ever (rarely under 30 FPS in main FS2 campaign with mediaVP's). My guess is that your build's performance on FS2_Open will be close to similar to mine, maybe slightly better in cases that require a lot of video memory (lots and lots of large textures used on-screen simultaneously), maybe slightly worse at times when CPU is the bottleneck of the system (lots and lots of particles and collision detection of multiple objects going on).