Did some digging and found this
http://dev.mysql.com/doc/refman/4.1/en/full-table.htmlUnless you are running with a very old version of Windows or Linux then the 2GB limit, so ignore the comment above from me. That Windows limit is gone now.
What may be more likely is that the database was usuable in an inefficient mode before but simply takes too long to do anything now. It is more than likely that you may need to look at ensuring that fields referenced in WHERE and JOIN clauses have INDEXes on them.
Conversely, if you are inserting lots of data into a table with a lot of indexes then it is usually quicker to lock the table, delete the indexes, insert the data, rebuild the indexes and unlock the table.
You will solve nothing by upgrading MySQL.
