Don't worry about that. The cache optimizations run in the Linux kernel and trust me, when it becomes necessary to cache the data, it will be cached. Run the system for a few days and see what then happens.
Also note that the system has not precognitive powers and it can hardly cache your whole hard disk, so when you run a program for the first time, it will not be cached.
If you find yourself with too much free RAM on your hands and a specific application that relies on ultra low access and read times, you can always construct a RAM-disk. Normally, it is hardly worth an effort.