You're right, in as far as that goes, but the CPU isn't the only thing in your system that needs to access the RAM. Look at the blue numbers on the chart on this page: dual-channel offers >50% boost in gaming performance of nForce2 with onboard video
. Obviously onboard video is the king of the bandwidth hogs... but wait, you say... my board doesn't have that crummy lukewarm onboard video, so what's the point?
Well, take a look at the blue numbers in the chart on this page
and you can see that in some specialized situations, the dual-channel feature still offers ~20% performance boosts, although probably not the stuff you do on a daily basis. The reason this happens is that, with all that excess bandwidth, the northbridge's memory controllers can afford to guess what the CPU will want next out of RAM, and pre-fetch it. If they guess wrong... no harm, no foul, because the excess bandwidth wasn't needed for anything else anyway. They toss the data and guess again.
The AthlonXPs already do their own data pre-fetching to some extent, so having the northbridge doing its own guessing on the side will usually not get you any further ahead, judging by the benchies there. But where it does pan out, it really makes a difference.
If you don't like having more bandwidth than the CPU can consume by itself, nVidia does have you covered... they make a single-channel nForce2 northbridge that you can find on boards without integrated video. Look for boards with the "nForce2 400" as opposed to the "nForce2 Ultra
400," such as the Asus A7N8X-X or the Shuttle AN35N non-Ultra.