According to rumor the second movie will blow the first one out of the water. It's supposedly good enough to be theatrical.
Also another movie idea in consideration might be a director's cut of the SG-1 pilot, maybe for the big screen.
If you're gonna get SG-1, splurge and get Atlantis seasons 1-4 as well (4 isn't out on DVD yet though). I enjoy both shows immensely, and a third show, Stargate Universe, is in the works which also sounds good. Maybe we'll see it materialize in 2009. If not then it might not show up until Atlantis ends its run, whenever that is.
[Edit: Oh you meant the original Stargate movie. Thought you were talking about the SG-1 movies.
If you end up picking up all of SG-1 get the first SG-1 movie too, The Ark of Truth. It wraps up the Season 9/10 story arcs. The second movie, Continuum, is then free to be awesome without previous story line baggage.
Interestingly, the guys who made the original movie reportedly weren't happy with where SG-1 took the story. They had a different vision which was realized in a novel trilogy or something. Something else to look for I guess.]