Yes, the higher the number the faster the speed. But it all depends on the processor you have, as the Front Side Bus (FSB) of the processor can differ, and also to the motherboard you have as there is a Memory Bus speed too (these are the speeds you have mentioned).
PC1600 is 200Mhz, PC2100 is 266Mhz, PC2700 is 333Mhz, PC3200 is 400Mhz
Look for the memory that best suits your processor, i.e. an Athlon 2200+ uses a 266Mhz FSB, so get PC2100. A P4 2.53Mhz CPU runs at 533Mhz, so buy the fastest memory you can get (PC3200) for the best performance.