Graphical LCD interfacing

If i am using a 640(RGB) x 480 graphical display. To refresh this i am using a 16 bit controller (a generic one, for now). This controller will be receiving data (8 bit ) from another controller, This data is to be proccessed and then an output is expected onto the screen.

If the data is being sent every 20ms then what should be the core frequency of the graphical controller. (the 15 or 25 frame per sec funda is not an issue here)

Could anyone help me out with the math. 'cause have got a value of around 4us as my core frequency. Am i right or wrong

THANX