If you are watching for the first time, it is highly recommended to follow the to keep track of major crossovers and plot developments between SG-1 and Atlantis .
Start with the SG-1 Season 8 premiere, "New Order: Part 1 & 2". Stargate SG1 - Season 8
This season features a major escalation and ultimate resolution of the Replicator threat. If you are watching for the first time,