System only recognizes 256MB of 1 GB- WHY??

Hey, I'm new here, and having a little trouble

So my system is only recognizing 256MB of the 1 GB DDR2.

Here's what Ive got
Intel Pentium D 820 (LGA 775 2.8 dual core)
ASUS P5VDC-X, it says it supports 2 x DDR2 533/400 to 2 GB
Single 1 GB DDR2-533, I think the brand is NANYA, the tag says DDRII/T 1G PC667 AX18V64oBT-3 0636 if that helps
http://www.smksuperstore.com/catalog/memory.asp?ID=368

You know what, now that I actually read that tag. It may be that I have DDR2-667, which my MOBO does not support (or supports to 256MB). But that website doesn't supply DDR2-667? Did they give me the worng one, or am I not thinking straight here

What modern MB only supports only 256MB of RAM?

Have you tried the memory in a different slot?

welcome to the forums.
where does it tell you that it only detects 256mb ram? if its in bios make sure you're not looking at video memory or something like that.