Linux is borrowing unused memory for disk caching. This makes it looks like you are low on memory, but you are not! Everything is fine!
How do I see how much free ram I really have?
To see how much ram is free to use for your applications, run free -m and look at the row that says "-/+ buffers/cache" in the column that says "free". That is your answer in megabytes:
$ free -m
total used free shared buffers cached
Mem: 1504 1491 13 0 91 764
-/+ buffers/cache: 635 869
Swap: 2047 6 2041
If you don't know how to read the numbers, you'll think the ram is 99% full when it's really just 42%.