The use of Tempdb depends on a lot of things: how heavely cursor, sorts, on-the-fly agregations are used, rollups, etc.  Also Tempdb is shared with all DB's and users, so as usual, it depends.

IMHO this what you can do: set the log file to 500 MB, and set both to autogrow with 100 MB increments. Then let it run for a week or so and then use the database's Taskpad view in Enterprise Mgr. to find out how filled both files are. And make adjustments from there.

Word of caution: every time you shutdown SQL, Tempdb gets deleted. And we you start it up again, its created empty. So make sure to check its size before you restart SQL.

Hope this helps.