Character or Numeric as PK

Hello,
I am from the school of thought that you should in every case have your primary keys as numeric values only. However, where I currently work there is a project leader who is a recent FoxPro convert (I know, they are tough ones to crack). I made the suggestion recently that the keys in the table should be numeric and with him being the project leader and me just a lowely developer he said get lost. I made the point that later joining your tables together in a PK/FK relationship where the keys where character would be slower then with numeric keys. He didn't listen and now we are approaching production with a database that is really just a bunch of text file. He said that with SQL 7 it doesn't matter if the pk is numeric or character. I disagree. But I need solid documentation to take to him and to the managers to convince them. If anyone out there could advise me on this. And if anyone could give me or tell me where I could find documentation on why even in SQL 7 there is a need to use numeric keys that would be a great help and you would be making one more shop in this world a little bit more technically sound :-) Thank you in advance for your help.

kc