Integer keys vs. Varchar keys

Hi,

What performance gains would an integer primary key have over a varchar
primary key, for table joins?

On a 100,000 record table joining with a 9,000 record table, there
appears to be no difference. Index Scan cost=0.00..4.51 in either case.

Would one expect it to make a difference with a 1,000,000 record table?

Thanks, David