Your video card it probably struggling. A 8400gs is not a gaming card in the least. I hate to tell people go buy stuff, but if you are going to be a PC gamer, you need to have the hardware to sustain it. Your CPU is MORE than capable, but your video card is really just meant for desktop graphics, photo editing, and maybe a HTPC at the very most.
A 4650/4670 is probably the lower card I could try gaming with in 2010.