LoL need some CPU power too.
Bitrate - 3000, max 3500 for twitch (10 mbit/s upload should do the job without problems).
128kbps audio for nice quality.
Resolution - downscale to 720p (1,5 - you are using 1080p) or even lower resolution, you have to try it how well it works for you.
30 FPS, maybe 25 if you get a high CPU load.
Select a faster preset to lower the CPU usage, it will not look good, but you may have to.
Your GPU is nice, it can run LoL + stream without problems, but your CPU is pretty old. I was not able to stream 720p 30 FPS League of Legends with 1080p native resolution with my old Phenom II X4 965BE @ 4GHz.
So for me it was just the right time to upgrade my CPU, got an i7 and i`m happy with it.
So you have to try it and check what will work for you or is it worth it to upgrade your rig.