file limits?

I have a customer that is currently using a shopping
cart which utilizes the file system as a database.
He has one directory which contains thousands of
small files (one per customer).

In an effort to persuade him to move the system to
a SQL server, can anyone offer me some numbers as to
the limits on files in a directory?

Are these OS dependent, or more specifically filesystem
dependent?

For instance, are there differant values for:

msdos
fat32
ntfs
ext2
ext3
reiserfs


v/r
-Mike