Alternatively, if you don't mind spending a bit of money, invest in a SSD for the OS and Apps.
Use 1 of the 500GB HDs for all User DATA/Files and the other as a back up drive. (If you do not have a drive or external for backups now)
If you do have a back up drive for images already, same thing only you also have the RAID 0 option for the 2 HDs for USER DATA
This would serve far greater performance improvements over adding more RAM.