G4 iMac 20" RAM upgrade...

I own a used iMac G4 20". It came with 256MB RAM in slot one (as from the manufacturer I presume). It was upgraded to a total of 512MB RAM with a second '256' fitted in slot two. I now want to upgrade the RAM again and have been told that only one of the two RAM slots is changable/upgradable. Is this correct? If so, which slot can I upgrade to? (I'm presumuing the second slot). Can I have a '256' in slot one and a '512' in the other - or do they have to be the same; two x 512s? Thanks.