For the record. I tested Mysql tables in Memory for store some information. And i tested PHP's APC (APCu) for store the same information.

For 58000 registryregistries. (varchar + integer + date).

1. Original information 24mb in text format (csv format).
2. PHP's APC uses 44.7mb of RAM.
3. Mysql's Table uses 575mb of RAM.

The table only has a single index so i don't think that it is the main factor.

Conclusion:

Memory table is not an option for "big" tables because it uses too much memory.