Memory upgrade for laptop

Question

got an ECS elite group laptop model green 320 with a 256 mb sodimm. tried upgrading with a 512 mb pc400 sodimm but the system keeps sending a continuous short beeping sound. is it incompatible? the 512 mb sodimm works fine with an acer laptop so i know it is a working memory module. the laptop is working okay with the current 256 sodimm but i need to upgrade to 512mb for obvious reasons. do i need to reset the cmos settings. i cant see a reset button, do i need to open the case? what is the most compatible spec for the sodimm?