I can confirm that there are no "special" or hidden settings in the BIOS of the GA EP45-DS4. Pressing CTRL+F1 in the main BIOS screen does not unlock anything at all. All available settings are accessible by default. And the BIOS has no memory management options that allow remapping or anything similar.
Anyway, I decided to keep the HD4870 but sent my GA EP45-DS4 back today. I will collect some more information over the week but most probably get myself a X38 based mainboard. But not a Gigabyte again. There were far too many things that annoyed me (the memory access problem, slow boot, several restarts after changes in the BIOS and the fact that it lost its connection to my firewall (or vise-versa) when I turned my box off).