Linux by default caches most frequently accessed files in memory.
If you run 'free' command in the shell and check "buffers" and "cached" field you will see how large portion of your RAM is dedicated* to disk content caching.
*dedicated: as long as your applications do not require that RAM, it will be used as cache. If more ram is needed, Linux will automatically drop caches to make space for applications.
Depends on your needs, but for example, if you are hosting a couple of websites of which only few pages are visited very frequently, then in my opinion the best strategy is to create cache on a regular filesystem. Frequently accessed cache files will reside in memory and will be accessed as fast as ramdisk (trust me, I've benchmarked this over and over again). Other cache files, used less frequently, will stay on the disk and you have some MBs of your RAM spared for something else.
If you really have to put everything in memory, then here it is:
### Create mountpoint:
### Add this entry in /etc/fstab:
tmpfs /var/ramdisk tmpfs defaults 0 0
### Command to mount it:
### Ofcourse, if your kernel supports tmpfs.