Well, with 7 characters and 5 million records, you don't need much of a character set. You could get by with the digits 0-9 and wouldn't have to revisit the issue until you double the table size. Adding just a few more symbols would greatly increase the space to work in, so Fletch's idea of using hex digits would be a good way to go. (That'd give you over 250 million records before you need to expand your character set.)

Personally, though, I think I'd just use upper-case letters without vowels. That gives you over a billion records, and leaves out the I/1, 0/O/Q, etc. confusions...