These tools have proven usable to me whne working with performnce problems on ubuntu:

htop (apt-get install htop)

top

vmstat

ps ax

free

Note that Linux likes to use any extra memory to cache hard drive blocks. So you don’t want to look at just the free Mem. You want to look at the free column of the -/+ buffers/cache: row. This shows how much memory is available to applications. So I just ran free -m and got this:

             total       used       free     shared    buffers     cached Mem:          3699       2896        802          0        247       1120 -/+ buffers/cache:       1528       2170 Swap:         1905         62       1843 

I know that I’m using 1528 MB and have 2170 MB free.