So I have bought another 512 MB SDRAM module for my iMac G3. Now I have 2 x 512 MB SDRAM Kingston modules inside.
After few hourse of using full capacity of RAM I can say it is a bit helpful. Geekbench score is higher about 6 points - 266.
Dashboard is still off because it needs a big amount of RAM and I don't need it, but with 1 GB RAM it can be turned on without a significant lack of RAM.
iMac is now almost in its full potential (no Wi-Fi card). It makes a good service for me. For easy browsing, listening, reading, watching - it is a truly good machine.