There's no "Memtest 64". Only Memtest 86 and Memtest 86+. I use the former of Memtest 86. Let that run its default passes which off the top of my head is four (4). If Memtest 86 spits out errors after ALL passes are complete then try one memory stick at a time. Memtest 86 will have an after report and display results in real time. If you get any error/s withen the first few minutes then you obviously don't need to keep running Memtest 86 for all four passes. You then need to weed out which stick is bad by testing one stick at a time. It may come to pass that you need to test each slot if it's the motherboard.
Make sure the memory is seated properly.
Make sure ALL memory is compatible. Right down to their serial numbers. I had four sticks of memory that were all the same model except one stick which was purchased latter had a slightly different serial number. But was the same model none the less. That one stick gave me issues. So, I bought a completely whole new set of four memory sticks that were ALL the same right down to their serial numbers. No issues.
Moral of the story: buy ALL of your RAM at once... Not some here and there even though they are the same model and Spec., etc.
You should never mix and match RAM models either. Some users do and can get away with it, but it may not work for everyone. The QVL (Qualified Vendor List) is something I try to follow form the motherboard manufacturer website, but it's more of a subjective list if you ask me.
As to memory brands. There really is only four or five flash memory manufactures that make all the RAM in the world. Two that are popular are Samsung and Crucial (Micron). I chose Crucial and have had no issues. Samsung is, well, a cult of personality.... Just like people and their use of Ccleaner and other crap... I lieu of Ccleaner there's the open source Bulk Crap Uninstaller. System Ninja is another that's great in terms of Temp file crap removal prior to cloning/imaging.
Anyway, I digress.
