Removing the heatsink on the CPU and the video card, cleaning it, and using arctic silver may work for a bit, but if there are any underlying airflow issues, it would only be a temp fix.
Yeah, that's why I said "for now". But usually, when everything was
running fine, and it starts gradually having overheating symptoms over time, it's a sign that there's an accumulation or build up of dust on either the case vents/fans and/or the CPU/video card heat sinks and fans or all of the above.
Another thing to consider, and I'm not sure about the card's capabilities, but to manually control the vid card's fan speed when you're playing your game. If the card is capable Dan, you can access the fan speed control in the Nvidia Control Panel under advanced/performance/device settings/cooling, provided you have the latest drivers. I use an external program to control my fans, but you can do it this way too. Again though, I'm not sure if your card supports variable fan speed control.
As far as adding case fans goes, yes it will help. Make sure you have an equal amount of fans pushing hot air out of the case as you do fans pulling cool air in.