You can run it. Things may take a little longer to do. Gausen-Blur, for example. Don't worry about it.
I think that Photoshop V4 takes up about 65 Megs for the program. You have plenty of room if you shut off other programs that are running along with Photoshop.
Just manage apps that are running.
Kidpeat told you that you could run V7CS on your computer.
My Mac G4 runs at 733Mhz w/1G ram. Not a problem at all.
Where did the 800 Mhz or the recommended Ram of 512 come from? Extra ram will help.
Inexpensive right now.
Try newegg.com. Good company to deal with. I have used them repeatedly. Very, very good service and tech help.
Nice to know that when you are ordering extra ram.
-Kevin