Memory

How do I find out what is the most ram I can put into my computer Emac?

eMacs support upto 1GB of RAM, using two slots that accomodate 128MB, 256MB or 512MB Dimms.

Please note there are different installation procedures depending on whether your iMac has ATI Graphics or not.

Source: http://manuals.info.apple.com/Apple_Support_Area/Manuals/specs/emac/L21330A_EN.pdf

More info:
http://docs.info.apple.com/article.html?artnum=88417 (RAM specs)
http://docs.info.apple.com/article.html?artnum=34860 (How to install)

I have oneslot with 256 can I add another 256 or 512

Yes you can add another 128, 256 or 512. You can also remove the old one and replace it with two 512 to get the most. It's up to you.

If I remove the old one I won't loose anything?

No, RAM does not store permanent information. It is only short term and clears itself anyway when the computer is restarted. Replacing RAM will not in any way effect the files stored on your computer.

this should eliminate some freezing? and speed up the computer when I have multiple graphics programs open?

It will definately speed up your computer while multi-tasking. There is a very good possibility that it will also make your computer more stable, as in preventing it from freezing as often. If you are having trouble with programs freezing you may want to try some maintenance first before spending money on RAM. Try a free program called Onyx to clean things up. However 256 RAM is not much, so you may want to upgrade anyway since going from 256 to 1 GB will make a huge difference.