PC2700 memory is DDR designed for use in systems with a 166-MHz front-side bus (providing a 333 MT/s data transfer rate). The "2700" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 2.7 GB.
PC3200 (commonly referred to as DDR400) memory is DDR designed for use in systems with a 200-MHz front-side bus (providing a 400 MT/s data transfer rate). The "3200" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 3.2 GB.