Free / Cached / Available memory on Linux

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http://serverfault.com – I have read that linux uses free memory for caching, to make system faster. However, both Nagios and Paessler PRTG monitoring system show me that my memory usage is critical. I could change Nagios mem_usage script to sum free and cached memory, but would that be correct information? I doubt that they misunderstood Linux memory usage. Lets say I have 8 GB RAM. 5 GB are used, 2 GB is cached, and I have 1 GB of free memory. Real available memory should be free+cached (3 GB)? If some new application would need additional 3 GB RAM, could it take everything from cache and free without using swap, (HowTos)