8400SE is a low-end card and not recommended for gaming.Upgrading to GTS 450 will give you a huge boost in performance.
And your CPU will fine for most games(what resolution do you play on? and what kind of games you play ? )
But definitely upgrade the RAM to at least 4GB
600w should be fine as long as it wasn't generic PSU. 2GB is bare minimum to have today. 4GB is the norm but you need 64 bit OS to utilize all the available RAM. below than that you will have trouble to run some program smoothly especially games.