upgrading an iMac G5 Power PC

I have a few questions about upgrading my pre Intel iMac. I recently upgraded from Tiger to Leopard 10.5.8. I can now sync my iPhone. However, I have noticed that my computer has slowed down. I want to upgrade my RAM memory. I have 512 MB DDR SDRAM. How high can I go and who is the best source for memory? What else should I upgrade while I am at it? I want to add Apple TV also. So whatever advice you can offer I would appreciate it.
Thanks.

Can you tell us a bit more about the iMac G5? Does it have an iSight Camera? What is the CPU speed and screen size and we can figure that part out.

The Last iMac G5's with iSight took totally different RAM than the 1st and 2nd generations.

It doesn't have the iSight camera. It's a 20" screen
And it uses DDR not DDR2. As for the speed I'll have
To check on that.