Hi margeryb,
You can see how much memory is being allocated by looking in the Windows Task Manager.
Unfortunately there is not a way to allocate a certain amount of system memory to a specific program, so the best option is usually to add more RAM to your computer.
How much RAM do you currently have?
Adobe requires 1GB of RAM to run the latest version of Photoshop.
My general recommendation is to have double the minimum requirements, which would be 2GB of RAM. The more RAM you have, the better your computer will be able to perform, especially when using very powerful programs like Adobe Photoshop.
Hope this helps!