[ale] Memory "free" vs what is truly available

[cfowler]

I'm trying to write a perl program that will monitor the output of
the free command and notify me when there is an issue with memory usage
I should be aware of

Output:
             total       used       free     shared    buffers
cached
Mem:       1026776    1018744       8032          0      89988
450292
-/+ buffers/cache:     478464     548312
Swap:      1044208          0    1044208

When should I be concerned.  Right now it appears as if almost all
memory is used.  There is 89m in buffers and 450m in cached Would that
memory become available if needed.  

I do not what to compare the columns total and used and notify then they
are almost equal.  That would not show a real problem.