I'm planning to by a Phenom II x4 955 BE and put it in an Asus M4A87TD EVO motherboard. I'd like to use DDR3-1600 memory, which Asus says is OK in that board, but I see that according to AMD the CPU only has 'support for unregistered DIMMs up to PC2 8500(DDR2-1066MHz) and PC3 10600 (DDR3-1333MHz)'. So what will happen if I put DDR3-1600 in?
The motherboard supports up to 2000MHz RAM so the 1600MHz should be fine regardless of the processor. I think 1600 & 2000MHz RAM is technically not supported by anyone or thing as this setting is technically overclocked and not a stock setting but as long as the motherboard is OK with it, it should be fine. You may have to set the speed & latencies manually though (I did with my 1333MHz RAM as the motherboard underclocked it to 1066MHz as default).
Agree with simon12. I've have that processor you're asking about, the 955 BE and run it with 1600 MHz ram. Using it with the m4a785td-v evo and it works fine, the motherboard box actually says OC 1800+ so it encourages you to OC. As simon said, 1600 is an OC'ed rate so no one will support it. Legalities and stuff.