why does my ddr3 1333mhz ram show up as 400mhz ?

Solution
Well there are 2 things here. DDR stands for double data rate. Therefore, when you run CPU-Z, it will show the speed of the RAM before it is doubled. So right now, your RAM is running at 800MHz effective clock. The first thing I would recommend is to go into the BIOS and check to make sure it is set to 1333Mhz.
Well there are 2 things here. DDR stands for double data rate. Therefore, when you run CPU-Z, it will show the speed of the RAM before it is doubled. So right now, your RAM is running at 800MHz effective clock. The first thing I would recommend is to go into the BIOS and check to make sure it is set to 1333Mhz.
 
Solution
What cpu or RAM do you have?
Because some of RAM will default to run at lowest speed at 800mhz, so you need to overclock the RAM, but it depends on what cpu you had, because the RAM speed related to the CPU (FSB) too.
Check the manual page 2-9. for more details. https://www.asus.com/Motherboards/P5G41TM_LX/HelpDesk/