guitarer

I'm having a wierd problem with my memory. I have 3 sticks, two 512s and one 1GB, all are PC3200. I have tried every combination of stick and slots, but no matter what I do, if all 2GB are in, the computer sees it as 1600. With any other amount of RAM less than 2GB is comes up as 3200. Any idea what might cause this? I have an Athlon 64 3000+ with a Shuttle AN50R mb. Thanks in advance.