Your MOTHERBOARD dictates how much and what kind of memory is supported.
My dad's Sempron 2800+ only supported 32-bit OS and his motherboard only supported DDR (not DDR2 or DDR3) up to 512MB per slot.
*Your Sempron may be 64-bit and your motherboard details may be different. If you have a 32-bit OS (like Windows XP 32-bit) you'd still be limited to 4GB of addressable memory (including the DDR and Video RAM).