Bios Version 088 adds support for DIMMs with 2Gbit SDRAM density. You will need to update the BIOS to the latest version 088 to use the 2GB modules. You can use a single 1GB stick to update the BIOS. You may need to update the BIOS versions gradually and not go straight to version 088.
If the current BIOS version is 0070 or earlier, you must first update to version 0085 before you can update to 088.
Thank you nails and Rodrigo for your answers!
Nails, you were right about the BIOS update. Shame on me, couse I've thought of it myself, but I misinterpreted the BIOS' name for the version. I thought that "86A" in RKG4310H.86A is actually the version. I simply made the error for just looking at the latest BIOS version in the list of BIOS updates and compared it to the one on my MB (which naturally is the same).... thanks to your post, I took a second look....