A memory hole of 512MB

Recently I installed 4 time 1Gigabyte memory in my Dual Opteron at home... I started it up... and the BIOS showed the 4GB (so I was happy), after that Gentoo got booted.

After that Gentoo was booted... I missed 512MB of memory. First I thought it was a Linux problem, but it wasn't. So I did a major upgrade of the BIOS... and hoped it works... but it didn't. After some google searched, it seems to be a Tyan Thunder K8W problem.. but there are solutions... but didn't figure it out yet