The PC???? refers to the rated performance of the memory and in the case of SDRAM generally refers to the MHZ at which the memory can run at such as PC66 runs at 66mhz, PC100 runs at 100 mhz and PC133 runs at 133 mhz.. there is available memory that is rated higher than this such as PC166 however the last 'official' SDRAM standard was PC133.
It may be worth remembering that just because the memory is rated at a specific speed it does not mean that you will be able to achieve that with your system as other devices may hold you back.
DDR memory offers greater bandwidth over traditional SDRAM and does not cost a great deal more. The rating method of DDR features below and works on possible bandwidth
PC1600 = (100 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 1600 MB/s available bandwidth.
PC2100 = (133 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 2128 MB/s available bandwidth.
PC2400 = (150 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 2400 MB/s available bandwidth.
PC2700 = (166 MHz Operating Speed) x (2x Rising & Falling) x (64-bit Bus) / (8 bits per byte) = 2656 MB/s available bandwidth.
