Highest temps I usually see from my GPU are in the mid-high 60s, but you should be safe well into the 80s, or even 90s, as far as what's actually safe. Most cards' "Tjmax" (thermal junction max) is about 100 C, meaning that if it hits that temp it will shut itself off for safety. You shouldn't have to worry about temps, as long as you keep your laptop on a solid surface that allows airflow underneath when playing games. Heck, I even play games on mine in bed, and it never heats up to unsafe levels. If you're really worried, there's an option in GPU Tweak (check your tray) to force the fans to max speed.
It should also handle all the games you mentioned just fine. As for Terraria and Mark of the Ninja... Right click on your desktop and go to NVIDIA Control Panel, then click on "Manage 3D Settings" on the left, and the "Program Settings" tab on the right. Click the "Add..." button next to the game drop-down and find Terraria and Mark of the Ninja - if they're not there, run them and then click "Add..." again. After you add them, check on the "preferred graphics processor" drop-down - make sure it says "High-performance NVIDIA processor." If not, it's not using your primary graphics card. It does that to save power. Keep that in mind for the future for any game that seems to be running slower than it should be.
