You dont have much choice, your cpu is probably too slow for x264 and games.
Use nvenc.
Assuming its twitch use 720p 30fps 2800 bitrate, or go for 3500 bitrate for best quality you can get on twitch as nonpartner.
always use vsync in game for streaming/recording to avoid stuttering!
also keep in mind that game should run with stable 60fps, only then obs will be able to capture 30fps without stutters, depends what game is it, but it may be not possible on 750ti