Stargate SG-1 will end, although some say it eneded a while back when 'MacGyver' started spending less time in front of the cameras. It was actually meant to finish a season or two ago (and possible move to movies). It's basically come to the end of it's run and is going out while still fairly popular.
Stargate: Atlantis
will be back for at least another season, maybe more.
Stargate: Infinity (the animated show) was cancelled a while back. I only saw a couple of episodes, but it didn't seem THAT bad and was better than most of the PokéBall Zero rubbish shovelled at kids these days.
There are vague rumours of another spin-off show. There is also plan by Dean Devlin to make two movie sequels to the original Stargate movie,
but he wont be bothering to attempt to integrate the Stargate TV shows.
This is also about to become a problem in the Star Trek universe, where the current idea for the next movie (by Lost and Alias creator JJ Abrams) is to "reboot" the franchise ... i.e. ignore everything that's already established and start again. The movie is said to be about Kirk and Spock at Starfleet Academy, and they want Ben Affleck to play Kirk.