I have a Dell OptiPlex 755, 4 slots on Mobo. Two slots are each filled with 1GB DDR2-6400 non-ECC, and the timings are 6-6-6-15 according to memtest86+.
Today I tried to put two OCZ 512mb's in slots 3 & 4, and the Dell freaked out. The timings are 4-5-4-15. I assume that mixing the two different timings cause the Freak. By "Freak", I mean that the Mobo did an alarm sound and wouldn't even attempt to load to the BIOS.
BTW, I did run the Dell with the OCZ only and it was fine and passed MemTest, so the ram is OK.
So I'm on the hunt for ram for slots 3 & 4. The QUESTION: do I have to find ram with identical timings? Would 6-6-6-12 work along side of 6-6-6-15 ram? I see the 6-6-6-12 a lot on Fleabay.