check the threads regarding "bodies" and "heads" in this forum for more info, including photos.
as for what kind of body i use, it really depends on how the neck is attached to the figure. if it has a long neck, like episode 2 anakin, a 21st century body is best used. if it has a short neck like jango fett, a hasbro body is best.
as for those GI Joe boxed "grunts" on clearance for $5, they have the same body as the new jango fett. 21st century sells their carded figures for about $10, which is a pretty good deal. hasbro used to do this, but since the release of their super joe body, they have added a few crappy acessories and jacked the price up to $15-18, depending on where you get them.(i suggest target for the best price on carded super joe's)
as for the thin arms, you really won't be able to tell when the figure is dressed.