But seriously, when you exhaust the density of your RAM, your apps still need the memory. When this happens, your hard drive will start using Virtual Memory.
For example, Windows 7 64 requires 2GB just to run smoothly. Technically, you can still use the OS with only 1GB, but the OS itself will be lagging. Now, let's throw in other memory intesive programs, such as Photoshop. If you don't even meet the requirments for your OS, you certainly don't meet the requirements for your app. This can result in seeing the "Virtual Memory Low" warning.