Hello, please note that the Solid State Drive should give you a very good performance overall if you use a SATA III port and a family of Solid State Drives that support that data transfer speed, and the ISRT technology is recommended when you have a regular hard disk drive and you want to get a better performance using the Solid State Drive as a cache acceleration option, but the Solid State Drive already have a great performance by itself.
I'm not sure that it's possible to use a RAMdisk and a real disk in a RAID array. You wouldn't want to do that since the data stored in the RAMdisk would disappear anytime you turned off your computer and then the RAID would have to rebuild on the next boot. That would take a lot of time.
In the scenario you describe, there is no mechanism to store the most commonly used files on the RAMdisk. That is not what RST does. It simply manages the RAID and RAID has no knowledge of what programs are most commonly used.
I think what you really want is a program like SuperCache. It does the work of tracking what programs and files are used most frequently and keeps them in a RAM cache (different than a RAM disk).