i means the fastest it can run ram at (without overclocking) is ddr400. You can put faster ram in but it will be down clocked to ddr400, and you can equally put slower ram in, which will be run at whatever speed it should be run at (i.e. it will not be force to run at ddr400).
pc3200 is just another way of saying 3.2GB/s peak memory bandwidth, which is ddr400 ram