Memory issues on ASUS P5Q3

Hey, so I've got this ASUS P5Q3 paired with an Intel Core 2 Duo E8500. When I picked up this rig a few years back, it came with 4GB of DDR3 RAM—basically just two 2GB Corsair sticks running at 1333. Lately, I've been doing more photo editing and graphic design stuff, and man, the machine is really starting to choke whenever I try to run two or three programs at once. So, I figured, why not just grab another 4GB? A buddy of mine runs a shop dealing with this kind of hardware, so he hooked me up with a single 4GB stick of Corsair DDR3 rated at 1600. He told me it’d work fine, even if the motherboard had to throttle the speed down to 1333. Well, I popped it in, hit the power button, and... nothing. Just a black screen, like the PC didn't even know it was alive. I pull the stick out, and suddenly everything works perfectly again. So, I tried another one—this time a 4GB Kingmax stick, also 1600—and same exact story. Total blackout. Even when I try to swap my old sticks out for the new ones in different slots, it won't boot. The screen stays dead. Only when I put my old setup back in does it behave. Then, my buddy gave me a pair of 2GB Corsair sticks, both 1600, and when I throw those in, the PC runs just fine and actually shows I've got 8GB total. My question is, why the hell is it doing that? Look, I'm no tech wizard when it comes to motherboards and memory timings, so I'm just wondering why I can't get a single 4GB stick to play nice. For context, my board has 4 slots—two are blue and two are, I don't know, red or orange... some kind of mix.

Here's what the specs say about the board.
Front Side Bus- 1600/1333/1066/800 MHz

Memory-4 x DIMM, Max. 16 GB, DDR3 1800(O.C.)/1600/1333/1066 Non-ECC, Un-buffered Memory
Dual Channel memory architecture
*DDR3 1600MHz or above DIMMs work only on the Orange slots for One DIMM per Channel.
*When installing total memory of 4GB capacity or more, Windows® 32-bit operation system may only recognize less than 3GB. Hence, a total installed memory of less than 3GB is recommended.

Like I said, I'm a complete amateur here, so I have no clue what any of this means. Can someone give me a plain-English explanation? What am I supposed to do? Because I gotta give these two working sticks back to my friend—they aren't even mine, they're from his computer.

Those different colored slots on the motherboard? That's all about setting up dual channel mode. Basically, you want to pop your RAM sticks into two slots of the same color to trigger that dual channel setup—it gives you much better memory bandwidth. If I were you, I’d look into getting four 2GB sticks, or honestly, it's probably easier to just grab two 4GB sticks so you can run them in dual channel and call it a day.

Thanks for the tip. I was actually digging through my motherboard manual yesterday and had the exact same thought. The manual says if you're running higher speed RAM, like 1600MHz, you’ve gotta use those >. Right now, I’m rocking two 2GB sticks, so that's 4GB total. Honestly, it feels like the smarter move would be to just grab two 4GB sticks and jump straight to 12GB. It’s gonna sting my wallet a little more, but hey, it's probably worth the headache. I tried tossing a single 4GB stick into an empty slot earlier, but the screen stayed black—total dead end. So, looks like I either need to stick to pairs of 2GB or go all in with 4GB sticks... I'm gonna give it a shot and see if the magic happens.

Buy it, but maybe double-check that the RAM speed is actually 1333Mhz.

Look, the only silver lining here is that I’ve got a buddy who deals in RAM, so he hooked me up with two 4GB sticks just to see if they'll play nice with my setup. It's a low-stakes gamble—if they work, I buy them; if they don't, I just hand them back. No harm, no foul. Anyway, I was digging around online and apparently, you can actually run 1600MHz memory, but there's a catch: you have to plug them specifically into those Orange slots for One DIMM per Channel. Since his shop didn't have any 1333MHz stock sitting on the shelf, he just handed me these 1600MHz ones instead. Let's see if the hardware actually listens.

DDR3 1600MHz or above DIMMs work only on the Orange slots for One DIMM per Channel.

If I'm reading this right, you can use the orange slots, but maybe only one stick can run at 1600.

So, yeah... bad news. Testing results are pretty much a disaster. It’s just not happening. First, I tried popping two 4GB sticks of 1600MHz RAM into those orange slots, and—nothing. Dead air. Then I tried running just a single stick by itself? Still nothing. But then, get this: when I swap them out for two 2GB sticks at 1600MHz, the damn thing actually boots up. Honestly, I can't wrap my head around why.
I mean, I doubt it has anything to do with brands, though... the stuff that won't work is Kingmax, while the stuff that does run is Corsair, which matches what I usually use. Even when I tried a 4GB Corsair stick, it was still a total bust. This specific setup in my rig uses Corsair xms2. At this point, all I can really do is sit tight and wait for that 1333MHz kit to arrive. Only then will I know if we're actually getting somewhere.