The last time I ran into this I found an out of date BIOS. One of their fixes was to add support of more memory modules.
Since your machine is fairly new, why not have them update the BIOS?
I upgraded my RAM from 256 MB to 1GB in the form of two Kingston DDR 512MB modules. My main board is Elite group K7AMA with a 1 GB max and two DDR slots. If I install both modules it doesn't boot, just gets to the boot menu (sometimes). If I use either module alone in either slot it works fine. I returned the modules to Kingston which they were very prompt in replacing with a different build but these don't work either. What's going on? Do I need to change the jumpers [jumpers are greek to me] or the BIOS or what?