Why does my RAM say 400mhz when it should say 800mhz?!!

HeLLpRoxXx

Honorable
Nov 25, 2013
53
0
10,640
I have:
motherboard asus asrock n61p-s
amd athlon 64 x2 5200+ 2.71ghz
Nvidia Geforce Gt 610 1gb
hdd 80gbb
And i have two ram memories,and they are 800 mhz,but cpu-z is reading is showing 400mhz,why my mobo is suporting 800mhz ddr2 ram,also in my bios i have for ram to choose auto,266mhz,533,and 800mhz..and its on auto but i don't is it safe to choose 800mhz :)
Thank you :))