For starters - please read this (the "RAM Optimizers/Defragmenters" section) to find out why using "memory optimizers" is a bad idea.
Unless you experience performance problems because of low memory there is no need to worry. What probably happens is the Vista's Superfetch mechanism attempts to load data into memory in advance so programs would work faster (for example it monitors which programs you usually run and loads some of their parts into memory). Additionally, the more RAM you have the more conservative Windows is in releasing data from it (since the data might be needed again and the process of loading data to memory is time consuming). More information can be found here (should apply to earlier versions of Windows as well).
How can you tell if ou have enough RAM? Check here (the "Do You Have Enough Memory?" section). In short download and run Process Explorer, click View -> System Information and check the "Physical Memory" section.