OBS (Twitch): LoL

I'm going to assume you meant 1920x1080. Anyhow, enable VSync (to avoid page/screen tearing) and you should not have any issue whatsoever streaming in 720p @ 60 FPS. You can just use the downscale filter (Lanczos) to scale it down to 720p while playing it at 1080.

For OBS: i recommend: 2500. So, go into Settings -> Encoding -> Enable CBR -> Max Bitrate: 2500 (3500 is the unofficial maximum accepted by the Twitch.tv staff but is not really required for a MOBA.

Settings -> Video -> Resolution: 1920x1080 (still assuming that is what you meant) -> Downscale -> 1.50 (720p), set the filter to Lanczos (32 samples). Framerate: 60.

Settings -> Broadcast Settings -> Fill out as needed/required. You can find your stream key in your Twitch.TV dashboard (under Streaming Apps).

Try keep resolution and textures high. Keep everything else off/low. Shadows, Physics, Particle Effects for LoL.