Generally, any OS will use the L1/L2 cache RAM first, then the SDRAM, then the swap file. This is based entirely on speed. The OS determines/anticipates what will most likely be the next series of commands or data that it will be required to execute/access and loads it into the appropriate memory. The memory level is generally kept pretty busy and full during normal use.
There *might* be a way of changing utilization but I don't know of any, and there may be unpredictable consequences since the OS cannot allocate memory according to it's own code. I would leave it alone.
Hope that helps,