It is a succulent, looks more or less like an Aloe vera, but it is
several feet tall and around. It looks like it has a coating of
powdered sugar. And it is a species that can tolerate zone 7 winters.

Something like this?
http://davesgarden.com/pf/go/60610/
or one of these?
http://www.plantdelights.com/Catalog/Fall/page3.html

Here in the Washington, DC area (zone 7), some kind of agaves or
yuccas are moderately common, including in the house we bought. We
have a nice big clump of three or so plants. They send up a nice
flower spike with white flowers (especially striking at night). Ours
does not die after flowering (some do, although they might send out
additional plants first). But I don't know what species are most
popular or work best (or even which one is in my yard, for that
matter).

They are, of course, native to drier climates. But ours has required
no maintenance and seems to be doing fine.