Hi,
Very good to see an old computer is still up and running. You definiteley need more RAM for your machine but the max is 1Gb (or may be 2Gb with the right BIOS):
You can buy RAM from the followinf reliable vendor:
http://www.crucial.com/upgrade/HP+-+Compaq-memory/Presario/Presario+SR1111NX-upgrades.html
Regards.