Yep. You're heard right. Actually, both SC and BroodWar in one cartridge! You only have access to the one-player Starcraft Campaigns if you don't have an n64 with the mem expansion pack.
However, they had to cut some stuff out; because of the size limitations of the cartridge. Only the opening movie is a movie, and its really shortened at that. All other movies, except maybe the final ending for Brood War (haven't gotten to that point in the n64 version; I'm hoping they left it as a movie) are rather slide-show type cinemas that give the basic scope of the events in that particular movie.
Also, all briefings are given as text-only, no voice overs accompany it, to my disappointment. However, a majority of the in-game unit speech used when they're selected and given orders is kept. Unfortunately, the scripted in-game voice overs during missions is again given as text.