TBH, the 8400GS is not really a gaming card.
You can probably play quite a few of the more current games, just at extremely lowered settings and at a low resolution.
If you would like to game with your system, I would recommend saving for a newer GPU.
I believe that you could be ok with your current HDD even if you haven't said you have one(you do, right?). You can also keep that CPU. It ain't that bad. (a new one would be nice)
100$ card like outlw6669 said, 4 gb of ram, a new board, and a new power supply. I assume you already have the case.
If all i wrote above is wrong, ignore it then.
A card that costs that much needs a lot of power at load so you will need to have a good power supply also. If your current one is not that good, then your pc WILL CRASH!
Yes, the Athlon II x4 will be powerful enough to handle all current games.
If you decide you need a little more juice, it can also be overclocked handily.
At your resolution I would expect this build to do quite good with all current games.
Personally, I use headphones for most of the time.
You can usually get much better sound quality for a smaller price going this way.
At $35, the Koss Portapro are an excellent choice and really sound amazing for the price.
If you would prefer regular speakers, look into the $90 Logitech X-540.
It offers quite a bit of sound for the price.