Working out more often will make a definate difference. It is alright to start slow, get used to everything first.  You don't want to be injured so take it easy when starting. 

If you stay stuck at 320lbs though you will need to continue adding a few more workout days to the week **OR** making the current days you workout, more intense.

On top of that, if your diet is low in fat & carbohydrates , you will get some insane results. Good luck!