The problem is the 9400 GT is a weak video card for games. It is more like a multimedia card due to it's low performance. Based on the resolution of the screen shot, I am assuming you have a 1400x900 resolution monitor.
Below are benchmarks for Team Fortress 2. It uses the same game engine (Source) as Portal 2 so performance should be similar. Since 1400*900 resolution was not tested the best resolution to look at is 1280x1024 which actually is slightly higher in resolution than 1400*900. It would be best if you have any of the games used in the benchmark tests to determine how well your CPU and 9400 GT actually is rather than relying on a single benchmark.
im getting about 40-50 fps on portal 1280 x 720 everything maxed out, 16x q csaa and 16x anistropic filtering with the GPU usage being around 90%. the real issue i have is occasionally the performance will drop and the gpu usage will drop - almost as if it were a power saving feature.
believe it or not minecraft seems like very non-intensive game, but the optimization for it is very poor, so as the complexity of the world goes up and increases in memory so will your performance except it will decrease.
long story short, 9400 is not a very good card and if you're getting 30 fps from it, you should be happy
Agree with everyone above.
Minecraft performance issues are not system based. They seem to happen even on high-end ones.
9400GT is a weak card ( I think we all said that enough already).
And if you're getting 40-50 FPS on Portal 2 maxed at that resolution, you should definitely be happy about it.