How much is too much? a few Gigabytes for the "old" mysql, and a few hundred TeraBytes for the new mysql.
Data storage limits are not your biggest worry.
There is an article in the articles section taht describes "database normalization"
That will help you understand why and how you can split up your tables (if there is a need for that at all in the first place).
Remember, every time you split a table up, you'll have to join them back together in SQL, which is an "expensive" operation.
What you need in the end is a database structure that you can work with.
If that structure happens to be slightly slower than another setup, so be it.
Better a "slow" database than a corrupted one :-)