The more physical memory you have in the system, the more fast memory is available to Windows for its own use and for applications, especially multitasking. If Windows needs more memory than the physical RAM fitted, it has to resort to using the page file on the hard drive, performance suffers.
With 4GB of RAM installed, Windows will typically be able to utilize around 3.2 to 3.4 GB of that (but it varies between systems), some is not accessible because it would have to go where system hardware is located. If Windows is being asked for the full 2GB allocation by a large program, it can supply this from fast RAM, and still have 1.2 to 1.4 GB of RAM for its own purposes. This minimizes use of the page file, so giving best performance.
As the amount of RAM is decreased, ie 3GB, 2GB, less fast RAM is there to use, so Windows has to make more use of the page file when memory demands are high. So for memory-hungry applications, having 4GB of RAM will be a benefit.
Both Photoshop and CorelDraw will experience the same effect, depending on how much memory each program actually allocates.
Edited by Platypus, 17 December 2008 - 03:36 AM.