Definitely not worth doing for continuously writable databases - read only databases will only see a benefit.
But wait. Mysql and your O/S should be caching the files in ram anyway so there is no need for a ramdrive here. All you need to do is ensure you have plenty of ram and tune with something like
MySQLTuner by Major Hayden
Ramdrives should be used for storing legacy databases, processing of temporary files and indexes, session files, files you want to permanently cache which may not be cachable via the o/s or apache.
A few more inexpensive ways of increasing performance (assuming linux / apache / mysql)
Have two disks but don't mirror them. One disk is used for the o/s and the other just for the database files.
Set log files to absolute minimum. Don't log image files, .js etc.
Remove all unecessary services from starting up.
Turn off noatime on the volumes.
Set disk acoustic level to fast / loud.