>>14
InnoDB is not a "poorly implemented hack". Do you actually have any idea of what you're talking about? Many database engines in widespread use today can envy InnoDB's MVCC system.
>>15
Probably, but unlike PHP, MySQL 5 is fully backwards-compatible with MySQL 4 and upgrading from one to another is a snap. Basically, you get extra performance (and more features to use in future development) at no cost of any kind.