If you're on the New Google Sheets, this is all you need to do, according to the docs:
change your recalculation setting to "On change and every minute" in your spreadsheet at File > Spreadsheet settings.
This will make the entire sheet update itself every minute, on the server side, regardless of whether you have the spreadsheet up in your browser or not.
If you're on the old Google Sheets, you'll want to add a cell with this formula to achieve the same functionality:
=GoogleClock()
EDIT to include old and new Google Sheets and change to =GoogleClock()
.