What does it mean when a motherboard says it only supports dual channel? My mobo says that, but it has 4 RAM slots. Does it mean that I can put in four but if I only put in 2 I have to put it in the same-colored slots and I can't have just 3 sticks?
Dual channel is the name of the technology. Configuring your RAM placement in your mobo is how you achieve/activate dual channel. The use of paired, not necessarily matched is one of the requirments. See below.
Rules to Enable Dual Channel Mode
To achieve Dual Channel mode, the following conditions must be met:
Matched DIMM configuration in each channel
Same Density (128MB, 256MB, 512MB, etc.)
Matched in both Channel A and Channel B memory channels
Populate symmetrical memory slots (Slot 0 or Slot 1)
Configurations that do not match the above conditions will revert to Single Channel mode.
The following conditions do not need to be met:
Same timing specifications
Same DDR speed
Memory channel speed is determined by the slowest DIMM module populated in the system.
*This really depends on your motherboard. Some manufacturers use a color
-coded system (meaning use the same colors) and some require that you use the symmetrical approach (meaning same slot on both channels).