What two factors must be considered when replacing old RAM modules in a PC?

When replacing old RAM modules in a PC, it's essential to consider the compatibility of the new RAM with the motherboard, specifically the type (e.g., DDR4, DDR5) and the maximum supported memory capacity. Additionally, the speed (measured in MHz) of the new RAM should be compatible with the existing system to ensure optimal performance. Lastly, it's important to check the form factor (e.g., DIMM for desktops, SO-DIMM for laptops) to ensure a proper fit.