I actually have quite a bit of experience. I dont personally use photoshop so that may have a option to reserve memory but that is by no means a common option. Its also something thats not needed as any program can use all available system memory at any time. Thinking on it now i may have come accross a aimilar option in a few programs meant for art creation but the option was just there to prevent crashing and lag due to storage in virtual memory. The point still stands though if you dont have enough available memory close uneeded processes and or buy more capacity. Work within the limits of your system and have as little running in the background as you can and hings will be fine