I think he means if you can keep your RAM running at full speed for best performance. Some mobos do allow you to run the memory bus at a slower speed if that is causing you stability problems.
I just checked price watch and the 1.6a is about $50 to $60 less than the 1.8a Northwood. However, at 2.4 gig the 1.6a will be running at 150 FSB and if you have the RDRAM setting at 4X (the max) that means your memory bus is at 600 MHz (600X2=1200). That ain't going to happen with the current PC-800 RDRAM. An Abit TH7-II mobo will allow a memory setting of 3X which would give you a memory bus speed of 450 MHz.
Now there is an advantage to using the 1.6a gig if you are trying to go for extreme overclocking and high FSB. In that case, even at 170 FSB (1.6a @ 2.72 gig) the memory bus could run at the "300" setting for 3X170=510 (or PC-1020). Yep, this is strong possibility with good Samsung RDRAM and good cooling.
However, once the PC-1066 RDRAM is introduced within a couple months, the advantage switches back to the 1.8a again, because then if you run the 1.8a over 133 FSB (greater than 2.4 gig) you'll still be able to run the memory bus at full speed if you have the faster RDRAM. That should give you something to think about.