I just tried to migrate an old MySQL 4.0 installation to MySQL 5.0. Most of the dumps worked smooth but there was a MediaWiki dump which raised the following error:
ERROR 1071 (42000) at line 66: Specified key was too long; max key length is 1000 bytes
Basically your new 5.0 DB defaults to utf-8 encoding. Type
STATUS; in your mysql shell to find out. Your old dump is most likely latin1 encoded and there is a problem when you import it and your index keys are longer than 1000 byte. This is because MySQL 5.0 uses 3 bytes for every character.
The solution to this is that you edit
/etc/my.cnf and insert /set the following lines: