My computer doesn't boot at all after adding new RAM

Hi. So I've got a problem with RAM. I have 4 DDR2 slots (2 yellow and 2 black ones) on my old Asus P5PL2-E mainboard. I've been only using the yellow ones until today with 2x1Gb Samsung 533Mhz RAM modules. Today I bought another pair of Samsung RAM, same size, but they're working on 800Mhz. I put them into the black slots, then the computer didn't boot at all, only black screen. After that I put the new memory modules in the good ol' yellow RAM slots, and left the old modules out, and it works perfectly.

So my question, could the problem be the frequency differences between the 2 pair of RAMs, or maybe the black RAM slots, that weren't used at all before, could be faulted? Both RAM modules work perfectly, so that shouldn't be a problem. I've tried to find some related settings in bios, but nothing at all, so I don't know what to do now. I appreciate any help!

You should -only- every buy RAM of the same exact type, size, everything. The more you change, the more it doesn't work. Any higher frequency will be changed to the lower one anyway automatically. Not a good idea o.0

That is why it doesn't work. Hope this helps

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