<Great info, thanks! I'm trying to wait for the Intel towers and a native version of Photoshop because my large file sizes with layers etc., should run much faster. For now, its a struggle. Kind of like this: Open file, make dinner, see if file open, OK, good, apply gaussian blur, eat dinner, see if blur completed, no, call long lost relatives and talk for hours etc.>
Something is very very wrong with your G4! It sounds like you have a Mac Plus! I open a 450+ MB file in less than 30 seconds in Photoshop CS2 on my 1.25 GHz G4. Gaussian blur take me less than three sceonds. When you print the file goes to a print monitor file on the HD and is sent to the printer as the printer needs it. A 14 X 21" print takes about fifteen minutes to print on my 4800 at the highest quality setting.
Check to see if all your installed RAM is indicated in "About this Computer" window. You might also want to check Disk Permissions. Go to your Applications folder and scroll down to Utilities. In Utilities folder locate Disk Utility. Open and click on your HD in the left pane. Click on the First Aid tab at the top. Now click on Verify and Repair Disk Permissions. This sometime speeds up the computer.
Another area that may need to be repaired is disk fragmentation. You will need some sort of external HD utility to perform this function. But first, back up your all you files.
If your G4 is 500 MHz or more, it is worth taking it to a reliable repair facility and have it chacked out.