Power Mac G4 help - running slow, no room, etc!

Part of the slowness is that you are almost out of drive space. Bad news when you don't have enough space for your RAM swap files.

Also when the drive is full, you are working on the inside tracks, which are the slowest, so drive performance takes a big hit

First step - have you Emptied your trash?

Then, look for an application like Whatsize on http://www.versiontracker.com and start cleaning up your drive. If you are running OSX, then also get DeLocalizer and delete unwanted language resources. This can free up to 1 Gb of space.

Burn a CD backup of your data, and start throwing away music and video files that you don't use anymore.

Since we don't know how much RAM you have, or the speed of your machine, or the model, we can't comment a lot more. Please provide more detail about your setup.

But Photoshop likes 1.5 Gb or more RAM AND a bunch of hard drive space.

Maximize the Ram, Minimize the Boot HD space used.

I would be getting a good storage drive externally setup, or even 2 to mirror backup.
500 GB - 750 GB drives are insanely cheap now & theres no excuse to need disc space.
Nothing works better for me to regain system speed than to:
Backup everything important & Erase the drive, then Re-install the system software.
Then only install the programs that you use the most, Leave the non crucial programs off. Repair permissions if running OSX.
Allocate a large ram usage to Photoshop in preferences, & use the external for scratch disk space.
Should cure what ails ya.
Did I mention to Max the RAM?
Then again you might want to get a new MacPro Octo, & be good for the next 5-8 years.