If u have a non-mission critical application running and u value performance, go for DDR400 non-ecc. ECC or Error Checking and Control is for stablity paranoids who don't want to lose a bit. With today's ram technology, such an occurence is extremely improbable at stock speeds and temp.
And it is DDR400 or DDR SDRAM @ 200MHz or PC3200 ram. (Not a ddr 400mhz ram)