Upgrading my old hp 510n

[Shinobi]

Hi, I was just about to upgrade the memory on my HP 510N. It only has 256MB or RAM at the moment. I wanted to add more. So I went on the HP website and looked up my model, it says it can hold a maximum of 512MB. Now I am running WindowsXP, and thought that I would be able to have more than 512MB. Is this 512MB max limit set by the hardware ability? Also, the sales associate at my local computer outlet sold me a PC133 512MB stick of RAM. Could I actually use just the one stick instead of two 256MB sticks of of SDRAM. I ask because if I read the HP site correctly, I think I am only able to have a max of 256MB or RAM in each slot. The type of RAM my computer uses is labeled as PC100 SDRAM. Any help would be nice. Thanks.