Linux is very lightweight. Even with an X server running and many apps open, you will not notice much disk swapping. Note that the 6 command terminals only use the memory of one, since Linux is very good about utilizing shared memory. Linux is very secure.
Every line of code is open to the public.
Several people who really know their stuff review each line of code that goes into the kernel. If a problem is found, and problems to happen, patches to the code usually come out in less than 24 hours. Compare this to weeks for a Windows NT service pack to be released. This may be one of the reasons NT earned a low security rating from a large auditing firm. Similiarly, SGI Irix has been notorious for security problems.