System expanding to use all RAM when idle

Question

Just had my Mac Book at the Apple Store for tests because the fan was running a lot & computer was running really slow.  They found no hardware problem but suggested my problem was that the 2G of Ram was not enough because the system with nothing open was using over 75% of the RAM.   Installed 4G of RAM, now 2 days later, having restarted the computer several times, the system has expanded to using 80% of the RAM with just Safari open.   Anyone else having this problem.. open to suggestions.  Have reset the PRAM, Reset SMC, ran long harddrive test, had it to the Apple store, had them run there tests on??  This is a 13' Mac Book (late 2007) model, running Snow Leopard 10.6.8 with UTD software, now with 4G of RAM using 3.81G of RAM with just Safari and the Activity Monitor open.   Thoughts, suggestions, cures ..  all would be appreciated!
Thanks!

Answer 1

You can find out which application is eating up your memory in Activity Monitor. Selet "All Processes" from the drop down on the right side of the window (next to the search field). Then click on the header for the column marked "Real Mem". This will list all of the process running sorted by how much memory they are reserving, and will give you a clue as to where the problem is.
What web pages do you have open in Safari? Safari can grab large amount of memory depending what kind of plugins a particular site uses.