The Memory Control Panel is available for you to set how your system handles some memory issues.
Disk Cache - This is a part of the memory your system uses for temporary storage that can be accessed very quickly when needed. Typically the default setting is the optimum and should be your choice.
Virtual Memory - This is a way for you to expand the Random Access Memory (RAM) available to run programs. It uses hard disk space to act as RAM. Due to the way that programs use memory setting the Virtual Memory to 1MB more than built-in memory (default setting) will allow programs to run using less RAM. You can expand Virtual Memory to 50% more than the built-in memory if need more RAM to run all the programs you need to have open for a task. Some applications (graphics and video in particular) have problems with Virtual Memory so if you are experiencing trouble with a particular program, try turning off VM and see if that solves your problem.
RAM Disk - This uses RAM has a hard disk which allows for very fast access to the information there. It is useful for programs which constantly write to disk. Do realize that this information is TEMPORARY and will be lost if you experience a crash and also uses the RAM you have available to run programs.