As far as I am aware, Mac OS X 32-bit processes (such as Photoshop CS) are limited to 4GB of RAM (not 2GB, as Windows is in 32-bit mode), so you don't actually have to do anything to get Photoshop to make use of as much RAM as it is able to. However, if you happen to have 8GB of total RAM, Photoshop can't use more than 4GB of it. This is not a limitation of Photoshop, but of 32-bit addressing.
Okay, thanks, that's clearer. I dropped back to CS because the Kekus LensFixCI plugin makes my PS CS3 crash (if it's not a flattened file) and the older, previous version of LensFix makes CS2 crash. Have used the plugin in CS and it's not crashing. I'm wondering if I had the memory usage set too high in CS3 and CS2. If the apps were trying to grab too much RAM (more than 1.5/6GB), and the OS was saying, "no, you can't have that much", PS shuts down. Is that possible? But then wouldn't I just get an 'out of RAM' message?