Yes. However, if you have a system with both PC1600 and PC2100 modules, the system will only run at a speed of PC1600. A system will only run at PC2100 if all the modules in it are PC2100 and the system supports PC2100.

PC1600 is designed for use in systems with a 100MHz front-side bus (effectively 200MHz in DDR mode). PC2100 is designed for use in systems with a 133MHz front-side bus (effectively 266MHz). However, since PC2100 was designed to be backward compatible, it has essentially replaced PC1600 in systems with a 100MHz front-side bus.

In order to qualify as PC100, PC133, etc., a memory module must meet industry standards for use in a particular type of system. In SDRAM modules, the numbers that come after the "PC" refer to the speed of the system's front side bus. In DDR modules, the numbers that come after the "PC" refer to the total bandwidth of the module. Here's a short summary of each type. PC100 memory is SDRAM designed for use in systems with a 100MHz front side bus.

PC2100 DDR is designed for use in systems and motherboards which require a 133MHz front-side bus. While the system or motherboard may operate a 133MHz front-side bus, its effective front-side bus speed is 266MHz in DDR operation. To get the effective front-side bus speed of a DDR system, double the operating frequency. PC2700 DDR is designed for use in systems and motherboards which require a 166MHz front-side bus, with an effective front-side bus speed of 333MHz.

DDR memory has been manufactured to be backward compatible so you can add faster memory modules to your machine. For example, if you install a PC2700 module alongside an original PC2100 module the memory will work and be fully compatible but you will not gain any performance benefit, as the PC2700 module will be clocking down to PC2100 to run alongside the original PC2100 module installed. Dual channel refers to the DDR/DDR2 chipset on specific motherboards that have two memory channels.

Although these are all types of double data rate (DDR) memory, the varying numbers refer to the different speeds of memory your computer was designed for. Crucial no longer carries PC2100 (DDR266) memory, but PC2700 (DDR333) memory and PC3200 (DDR400) are backward-compatible with PC2100. Memory is designed to be backward-compatible, so generally speaking, you can safely add faster memory to a computer that was designed to run slower memory.