MySQL tmp and tmpfs disk I have mysql and i use a tmpfs disk that is 768 M. Mysql uses about 70% of the memory. There is about 1170 M available.

If mysql uses ram to the point there is less than 768; will my RAM disk be limited in size?
In my opinion, the RAM given over to a RAM disk would be better utilized for other caching.  If InnoDB, give it to innodb_buffer_pool_size.  For MyISAM, give a little to key_buffer_size, leave most for data caching, which the OS does.