well, first off, the SGA ships were battle cruisers.... if that sort of technology were to fall into the hands of the wraith, they could literally super-soak their hives inside of suns... and that would be VERY bad
It seems that this area of the ancient technology is pre-ZPM, so they used things like planet cores and suns to power their stuff
As to a replacement of BSG, no... SGU isn't, because it is tryin to get the best of both... the backstory behind SGU has to stay Stargate, but the way they are focusing the story on the characters is much more of a BSG design. Stargate has focused more on the story and mythology instead of characters... which is why i always loved stargate. Anyone can figure out a person, or relate to a person who is a character on a show.... but its so hard to directly relate to an alien history, and it forces us to imagine... which is what Sci-fi is about