I have a Asus P6T X58 motherboard with 6 slots and 3 channels. In my manual, it says that due to Intel spec definition, XMP DIMMs and DDR3-1600 are supported for one DIMM per channel only. Your P55 motherboard has only two channels. Is your ram 1600? and, does your motherboard manual have the same caveat?
Dual channel is set by the bios if you have two sticks of ram, one in each channel. The extra stick will operate in single channel mode.
my ram is 1600...the manual does say that. Does that mean 6gb wont work, only 4 or 8 gb of ram?
I think it means that if you use the default XMP definition to get 1600 speed, you can only run two sticks. You could try setting the speed in the bios explicitly to less, like 1333 or 1066. Your performance will not suffer to any noticeable extent on a i7 cpu.
Ram is not explicitly 2 or three channel. It is sold in kits of two or three dimms to be used in two or three channel motherboards. If you have an odd extra dimm, that should be OK, it will just run in single channel mode. The extra ram is more beneficial than a couple of percent processor power.