the 266MHz you refer to is a DDR number representing actual clock cycles of 133MHz. You COULD run your RAM assychronously, faster than the CPU bus, but that doesn't help much (and sometimes causes enough system overhead to offer worse performance). Dual Channel doesn't help much either, but will give you a very slight performance advantage (ie, unnoticable to you, but a few extra points in benchmarks). Since it is an advantage, you might as well use it. And your DDR400 would likely be able to run at SUPER low latencies at DDR266 speed, which WOULD give you a nice performance increase.
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