To further explain where they get the PC2400...etc. number from, it's a matter of calculating bandwith....
Example:
PC1600 = 64 bit (8 bytes, so you multiply by 8) x 2 x 100 MHz = 1600 MBytes/s
64bit is the bus of the memory
x 2 = Double Data Rate
100mhz is the... well, I hope you know what that is.
PC2700 = 8 x 2 x 166 = 2700 (rounded)
2700MB/s
etc. etc.