You should also have a decent internet connection as well. If you're planning on streaming in 720p, make sure to have at least 2x the upload bandwidth of the combined bitrates of your stream. I think a 16:9 aspect size on 720p would require around 2340 bitrate and let's say the audio requires 64, that'd be a total of 2400 Kbps upload as a minimum for the stream.
I wouldn't say the GPU is all that important when streaming Osu from all games, but it's just common sense that you won't be streaming it if your FPS is barely reaching 200 even without any other applications on.