Tomshardware.com is where I've seen such setup and benchmarked. Not to duplicate the results here but you need to think about how close a machine is to what Tom tests there. Let's say you have a 9GHz Tri-channel machine and Tom tested the 8.8GHz Tri-channel machine. That's close enough. Now you find your 6GHz dual channel but you wanted the 6.2GHz model. Still close enough to answer your questions.
Hope this helps,
I think I'm going to wait for Windows 7 to be released to order, but have been playing around with Studio XPS 435 configurations. There are options for either Dual Channel of Tri Channel memory. Is the Tri Channel much faster, because it allows for one extra stick per set, or something else! Would a set up with 3 Dual Channel sticks be the same speed as one with two sticks of Tri Channel?