I wanted to know more about how to look at a linux (Red Hat Enterrprise) in terms of memory ? Should you look at "real memory " free using top or free -m or should you pay attention to "CAched buffers"

For example lets take this output from "free -m"
utput:
bash-3.00$ free -m
total used free shared buffers cached
Mem: 15808 15130 677 0 113 6171
-/+ buffers/cache: 4845 10963
Swap: 2047 0 2047


So lets say when you do ps -ef you see ther are a few java process and combined memory usage is around 10Gigs. So question is

a) Where is the rest of the memory ? Was it used for cached buffers when copying over some big files?

b) Can I start say 2 more java process with 2Gig heap space and it will get memory from the "cached buffers"

Bottom line if you want a healthy system should you be worried if you dont have real memory but have a lot of cached buffers?

The cached buffers do not matter, since these will occupy as much of the remaining memory as required.

The sign of having enough memory is that the swap space is seldom (if ever) used.

what are the Virtual File System in linux which does not carry actual space? explain??