Well, I have bougth a new PC and in my Mobo's (ASRock ALiveNF6G-VSTA) manual it says it supports dual-channel memory and when I start my PC it says at the startup screen that my memory runs on a single-channel mode and in the BIOS it tells me that it's running only at 400MHz and not the full 800MHz and I can't change it nowhere in the BIOS... How do I get it up to dual-channel mode? Thank you in advance.
DDR (and DDR2) are Double Data Rate memory. Unlike older SDR (Single Data Rate), data is transmitted on the rise and fall of the clock. The RAM does not operate at 800MHz, rather 400MHz, and due to the DDR thing, it's effectively 800MHz. As for dual-channel mode, this requires both channels to be filled with the same amount of RAM. Typically, and easiest way to do this is to have two identical sticks of RAM and place one in each channel. For the specifics on which slots correspond to what channel, consult your motherboard manual.
