If your system is designed for accepting DDR (Double Data Rate) memories of 184pin DIMM (usually desktops) or 200pin SODIMM (usually laptops) built, for your system to fully recognise 1GB capacity per memory slot, you need to use 'Low Density - 64Mx8 config' 1GB module. If you use 'High-Density - 128Mx4 config' 1GB module, your PC may only recognise it as HALF the size at 512MB or most of the time it will not work.
How to tell if your 1GB module is a low or high density module?
All low density 1GB modules are made with 16 chips (8 chips on each side) using 64Mx8 device.
All high density 1GB modules are made with 16 chips (8 chips on each side) using 128Mx4 device.