Fallout 1 in my view is still a great game but Fallout 2 outdoes it by a giant leap. This sequel improves on gameplay, graphics, dialog, and overall mission structure while feeling like a game with much polish. Sure it runs on Fallout 1's graphics engine but there was and still is potential on improving it to the point where the community just about perfected both Fallout 1 and 2.
Make sure to play the first Fallout game or you're probably going to be quite confused!