Gigabyte GA-K8NSNXP Ram Problem

Hello.

I have an old GA-K8NSNXP (nForce3 250, CPU socket 754, 3 DIMM slots).
I flashed my bios to the latest version, F15.
2 DIMM Modules of Samsung 512mb DDR PC3200 400 mhz.
2 DIMM Modules of Samsung 512mb DDR PC3200 333 mhz.
I tested each pair of RAM on another motherboard without any trouble.

Each RAM Module is working fine when installed alone. But installing a pair of modules will have my computer go black screen with beeps on start up (RAM error basicaly), whatever the socket combination is, and whichever DDR voltage is set up in the bios : auto, +0.1V or +0.2V.
If I try a combination of 3 RAM modules, I get no beep but the computer doesn't pass POST.

After a goole search and visiting the gigabyte support forums wich didn't bring any conclusive answer, I managed to get the 2 DDR 400 mhz modules to work together but at 300 mhz and +0.2V with manual DDR settings in the BIOS (at 333 mhz it would also beep). The rest of the bios is set to optimal settings.

I'd like to know if this currently working configuration might damage the hardware, or what do I exacttly lose from tuning the RAM down to 300 mhz.
Also I am interested in any information about this whole RAM compatibility problem with the GA-K8NSNXP, and any weblinks about it.