Today I was working on one of my photographic lens comparisons with three lenses, files from the 24-megapixel Nikon D3x.
Memory usage hit 23GB in Photoshop CS5. Now if that had been the new Nikon 36-megapixel D800, then it would have hit a whopping ~34 GB.
Which is why for my use, nothing less than 48GB of real memory will do. I use a 12-core 3.33 GHz Mac Pro with 64GB as 8 X 8GB memory from OWC. On a 4-core or 6-core Mac Pro, you can go to 48GB as 3 X 16GB.
See also my Optimizing Photoshop CS5.
You might be surprised by just how low prices have dropped on memory.
Check your memory usage to see if you have “Page outs”. If so, get more memory.