It should be compatible, but it won't give you full performance like a triple-channel memory kit would. It's a small difference, but it could be noticeable depending on what applications and games you run.
Triple channel never offered much performance over dual channel anyway! memory already has so much throughput, there was no need for the extra channel. You're better off with 8gb dual setup than 6gb tri setup.
The new i7's dont even use triple channel anymore, it proved pointless
Any amount of RAM over 4GB will give better performance for most users, but you should always use all of your memory channels if possible. I recommend 6GB minimum for triple-channel systems and 8GB minimum for dual-channel systems.
Having more channels isn't pointless, it is simply expensive. We're actually getting some extreme-performance desktop CPUs introduced in six months or so with quad-channel memory controllers. When other components catch up, having more memory channels will become more beneficial. We're starting to get to that point already.