^Umm, what's making it difficult to answer his question?
The buffering rate and pausing has all to do with transfer rates/speeds rather than the speed/memory of a GPU. Changing the graphics card won't do much if anything at all. Stream video qualities rarely require anything demanding. You should be fine with 64mb of memory. The only time you MIGHT run into trouble is using fullscreen for your streams. But even so, you won't be seeing "pauses." Instead, you'll just see more stuttering and slowness.