DDR1, DDR2, SDRAM? How to determine a PC's memory when upgrading

I have a desktop i bought in 2004. It's still working fine but I wanted to upgrade its 256 MB memory. But how do I know which type of RAM to buy? Without a manual, I took a look at the label of the existing RAM - it says there 256MB DDR 266MHz. But what is the maximum amount of memory I can insert?

I found this very useful application which can tell me what my RAM types are and more:

System Information for Windows by Gabriel Topala

www.gtopala.com

Note that each RAM slot has a maximum memory capacity and the motherboard has a total maximum capacity. It doesn't pay to insert 3 1 GB DDR's in a motherboard with a maximum 2GB capacity.