Don't use NVENC. I use an i7-920 myself and can stream 720p@60 Veryfast x264 no problem, on stock clockspeeds. Including FFXIV when I played it.
What CPU load are you seeing while streaming? You may have to quit some background programs (Chrome is notoriously bad lately). If you're recording locally to MP4, STOP IT. Use FLV instead, for many reasons (high CPU included).
If you are a non-partner, 720p@30fps, 2000kbps is the 'golden point'.
60fps streaming is mostly numbers-wanking (even if the 920 can do it just fine at 720p), and requires more bitrate than you can really afford.
You should definitely not be using 3500kbps unless you're a partnered caster.