|As the others have stated, the 8400GS is NOT a good card for gaming. It can't even keep up with many low end cards that are several generations old.|
You didn't list the complete specs to your PC, but 2GB RAM isn't very much for Win7, especially for gaming. And when it comes to video cards, you need to look at ALL the specs, not just the memory. Just because a card has 1GB doesn't mean it's fast or slow. 1GB memory is a complete waste on the 8400GS because there's no way the weak GPU can take full advantage of it. You need to look entire spec sheet - GPU core speed, number of stream processors, type of memory (DDR2, DDR3, DDR5), memory buswidth (64-bit, 128-bit, 256-bit, etc). Price is also a pretty fair indicator of performance. In the US, the 8400GS 1GB sells for about $20, what does that tell you?
Have a look at this benchmark chart to see how poorly the 8400GS is rated & notice the long list of crappy old cards that beat it:
You need more RAM & a decent gaming card. It's also possible that you'll need a better power supply. Read this article:
Best Graphics Cards For the Money: March 2011