I've faced similar issue, I have the same motherboard with same problem; try to downgrade BIOS version(presuming you have the latest .118) to .113 version, only then you will be able to install windows 7 without any hassel.
Sadly. this motherboard doesn't have 'MEMORY REMAPPING' option.
In regards to the last point, when you have RAM with same bus speed, you shouldn't have any problem.
Your motherboard will work great with 8GB of RAM, just try what I've recomended.
Please let me know, whether it resolve the issue or not.
Thanks & Regards,
This was exactly what the problem was. I down-gradded the BIOS to 0113, put in my new ram, and installed a clean version of Windows 7 64. Now I have a smooth running recording studio computer with 8 gigs of ram.
Thanks for your support.