Can i play these games on a 8400 GS geforce graphics card?

you can run most of them but it won't look pretty. the ones you can run will be on low settings. of course, if you want to fix this you need to buy a better graphics card and put it in yourself.

keep in mind that some stronger graphics card require a stronger power supply or else they won't work.

With this graphics card you should be able to play them. You should make sure that you have all other requirements needed to run whatever game you are playing. Also when you play you might want to consider putting the shadows off as they are unnecessary in some games (such as CoD, Bad Company, Medal of Honor, etc.) and will increase performance with them switched off. Hope this helps.

probably but u could upgrade to xbox 360