Something weird is going on!!!!

It all started when a friend and I moved our components into a new case.

MSI K9N Neo V2
Intel Core 2 Duo
2x1GB Ocz Platinum 6400
2x1GB OCS NVIDIA SLI
160GB SATA
AMD Radeon HD 6770

Everything was working perfectly in the old case. After moving everything over, the PC powers on normally, but there is no signal on the monitor. If I swap to the second monitor, it turns on, but once it boots, the screen is covered in colorful artifacts. If I pull two sticks out and leave just the ones in the first and third green slots, everything works fine and it boots straight into macOS. However, if I put all four sticks back in, the monitor won't even turn on. If I remove a stick from the second or fourth slot, the monitor turns on, but then the colorful artifacts appear, almost as if it were a video memory issue. It only runs properly with two sticks—it doesn't matter which ones—but they have to be in slots 1 and 3. I have tested all four sticks individually, and they all work fine as long as they are used in pairs in those specific slots.

I am a bit lost here. I really need to get my full 4GB back, because right now I'm stuck with only 2GB. I tried loading the BIOS optimized defaults, but it didn't change anything.

Are you certain everything is connected properly? 🤔
On the other hand, if you are—or if you think you are—try resetting the BIOS by removing the battery for a few minutes, then give it another shot...

I attempted a CMOS clear using the jumper, which should have achieved the same result, but nothing has changed. I'm honestly not sure what's going on. When I use all four modules, the monitor won't even turn on. With three modules, I get a distorted image filled with artifacts—little blocks and lines, almost as if the GPU is screaming. It only seems to work normally with two modules, and even then, only if they are placed in slots 1 and 3. It also works fine with just a single module.

Everything is connected properly: CPU power, motherboard power, hard drive, optical drive, and the 6-pin connector on the graphics card. All the modules are seated correctly; I've double-checked everything twice.

Clearing the CMOS via the jumper was the right move, but you did it without power!
And make sure all four sticks are actually seated when you perform that reset.
Once the jumper is back in place and the power is on, boot straight into the BIOS and select "Load Defaults."
Save and exit, then jump back into the BIOS to configure things properly—for instance, disabling any unnecessary turbo settings.
With older BIOS versions and RAM, you often have to manually tune the memory settings; not every BIOS plays nice with every module. Some settings can stay on Auto, while others require manual input. It’s just a headache.
It's possible everything was set correctly before, but now those settings are gone. You likely lost them due to a dying battery or a loose connection from when you moved the build to a different case.

For future reference, it’s usually better to stick with two sticks. Even if you install four, performance often drops because the chipset has to work harder managing four modules instead of two.
It’s a lot like dealing with women...
Two sticks are perfect for Dual-channel mode—much faster than a single stick, though on modern chipsets, the difference isn't nearly as significant.

Good luck.

I have tested everything, and now a friend gave me an AMD Athlon II X2 7750. Technically, this motherboard isn't supposed to support it, but since there is a newer BIOS available, I am going to try it. I actually found some discussions on US forums suggesting that people have successfully run this specific processor on this board before.

I am planning to install a different processor to determine if the issue lies with the memory controller or if there is an error within my current CPU itself. If that doesn't help, then I guess the dual-channel functionality on the motherboard is officially dead. At that point, we might just have to accept it and either hunt for a new board or switch to a 2x2GB RAM configuration instead of using 4x1GB.

We actually tested that Core 2 Duo in my friend's old setup where he was using this X2 7750, and all four slots failed to work there too. However, once he put the 7750 back in, everything worked fine again. I also checked his RAM, and every single module is perfectly functional.

I suppose I will find out later whether the problem is the memory controller on the CPU or something with the motherboard.

Problem solved. We ended up picking up a different processor and motherboard—specifically an ASUS m770e AM2+/AM3 setup with an AMD Athlon II X2 250 @ 3.0GHz for $83. My friend actually managed to snag a faster, newer chip that can be overclocked to at least 3.0GHz without any issues, plus it features a newer chipset that allows for a future upgrade to something like a quad-core Phenom later on.

Everything is assembled and running smoothly; all four sticks of memory are booting up perfectly🙂. He’s seeing a slight bump in gaming performance by a few FPS. We're planning to put the old CPU and board into his new rig, so if he decides to sell the upgrade later, it shouldn't cost much at all.

I want to thank everyone who tried to help out, especially those few who stuck with it. That's really all there is to it.