I have a question about RAM,

Well I am going to upgrade soon to 4GB because 1GB and vista really just like to take up all the RAM I have..

Now right now I have just Fire Fox open with 2 tabs. and I go right click task manager.. I have;

789MB used(about)... 1013 total... 308 Cached? and 14 free, well now it say's 7.. then 8... so it's basically staying below 15.. witch is bad right??

now, what in the world is Cached?? It's taking all my RAM!!!!!!

Can someone help me speed things up a bit??