Missing Ram

hello friends, i own a computer with asus m2n mx se plus mobo with amd5600+ processor. i hav 2 ram sticks of 1 and 2 gb(zion, 800 mhz) installed. the os winxp with sp2( i removed sp3 few days back) is only showing 2.5 gb of ram . also cpu z showed the speed as 400mhz on single channel. could someone please tell me whats wrong, or how can i recover the lost 512 mb of ram. is it for the onboard graphics. BTW i have an xfx 8600 gt graphics also..

please help me...!!

Thanks