It should work fine, but it's not ideal, and why not 2x4GB (unless your mobo doesn't support 4GB modules)?
As far as 6GB, in any configuration, being any faster than 4GB, it may be, but I doubt it would be noticeable. Higher RAM amounts hardly ever speed up a system unless you have WAY too little to begin with.
that's all I wanted to here.
thank you guys