We had tried this before switching to the memory table. I cannot remember exact details, but the resulting innodb table was still too large - maybe around 1 GB and we did not want to add additional RAM.

Another issue with using a second Innodb table would be the load time. Every 3 hours we empty the memory table and load the subset from the innodb table (we actually use an MyIsam table in the middle.. but that's not important). Loading the table takes less than 10 seconds. If we had used an innodb table instead of a memory table, this load time would be significant.