If you're forced to use single channel, you'd want your RAM to run 2x as fast as the CPU bus ideally. But in your situation there isn't memory for that (800FSB is 200MHz x4, DDR800 is 400MHz x2). It's a great idea for older CPU's with slower bus speeds and single channel boards.
If your board supports dual channel mode, it's best to have it synchronous to reduce overhead on the memory controller (800FSB is 200x4, DDR400 is 200x2, put that in dual channel for matched bandwidth).
The reason some people run their RAM SLOWER is because they're using RAM that's not rated for the speed of their CPU bus. For example, if you overclock a 2.8C to 3.5GHz, the "1000FSB" of 250MHz would require DDR500 to run 1:1. If you had PC3200 (DDR400) you'd need to use the 5:4 CPU RAM bus ratio in order to make the memory stable.
And getting back to your original question, running your RAM at DDR433 in dual channel gives you no performance gains, and is impossible with most chipsets. Most chipsets require 1:1 ratio for dual channel mode. DDR433 on such a board would run at DDR400 speed in dual channel mode. The point of DDR433 then is that it supports synchronous CPU bus overclocking.
Another advantage of the "faster" memory running at a "slower than rated" speed is that it's more likely to be stable.
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