Glide is a Graphical API.
Here is how I understand things, someone will probably correct me or say it better.
As I understand it, an API is a layer that interacts between Hardware and software. Programmers are able to code things for an API rather than having to code for individual hardware.
Microsoft?s D3D is the main graphical API. Initially D3D wasn?t very good and Glide became popular because it was stable and performed better (if you had a 3dfx Video Card). Later revisions of D3D have seen it improve significantly and it is now the default API that programmers will code for. However there are still a number of new software titles (games) that have been based on Glide. Notably Diablo 2, and Unreal engine games (inc. Deus Ex.). For playing these games, Glide is a superior option and as such isn?t dead. However, in the future less and less games will come out with Glide support and very few will support Glide exclusively (doing so will limit potential sales).
If you are buying a Video card, Glide support isn?t much of a plus, unless you wish to play the games mentioned above. It isn?t really a negative either, as any card that supports Glide will also support D3D. Pick based on its other attributes (Performance, FSAA, T&L etc).