Well, for one, I've never used EveryMac before until looking for a used iMac. While it seems to be authoritative, and I have no reason to distrust it, I've looked at the ads for over 100 used white 5,1 iMacs, and not one mentioned that it was "n" capable. Wouldn't you look for verification, perhaps from someone who owns one?
As for the second question, all hyperbole aside, it's a question of range, not speed. If possible, I would like to put the iMac on my farther-reaching "n" network, rather than my "g" network, which is in use by my TiVOs and other internet appliances.
But thanks for the info about upgrading if necessary!