Not to offend you, But most of the times this is the problem. If we post the doc/web link people say its just doc/web link, we need explaination. If we try to explain its not sufficient cuz, people don't read god damn documentation:( IMHO,Its very straight forward if you go to tahiti.oracle.com and pick interested version of documentation link you get a search window where you can search for init.ora parm you are interested in it shows exactly how to specify.
back to your doubt:
you can specify whatever value you want(required) in K/M/G (min value being 48M)there will not be any issues. In your case its going to be 40996*8=327968K So value for
db_cache_size=327968K or 320.28125M or 0.312774658G