MySQL: sysbench test - InnoDB vs Memory tables
I have addressed InnoDB vs MEMORY before


Jan 21, 2012 : Is InnoDB Engine up to speed against Memory Engine?
Jan 17, 2012 : Mysql Memory table getting many locks


I also addressed the one headache MEMORY tables have: There is still disk I/O because of the .frm file (See my post I am using the MEMORY storage engine but MySQL still writes to my disk...Why?).

Also worth remembering is the fact that MEMORY tables do table-level locks.

You should just increase the InnoDB buffer pool and leave MEMORY tables behind