Believe it or not, different games use different amounts of processing power. It makes sense that it was Heavenly Sword because that game puts the ps3's pedal to the metal. GTA4 probably uses a lot of power too, it was a long time in development not a port.

Most of the heat generated in the ps3's comes from the seven 3.2gHz processors in the Cell engine. It makes sense that a good game would generate more heat.

Your room temperature being 70 is fine. Keep it below 72 if you can. Does the fan get really loud, or just louder? It is normal for it to occasionally go from its lowest mode, so quiet you can barely hear it, to its next highest mode which is kind of like a vacuum cleaner running in another building.
There are more than 2 speed levels though, the highest is pretty loud, after that it shuts itself down. If it ever gets real loud, shut it down yourself.

Make sure there's enough space around it for heat to dissipate, even if it's cool enough in the room it could still overheat if there's things next to, on top of, or covering it. 4 inches on each side is what the manual says. It's a good idea to set it vertical too, if you can do so without risking it being tipped over. The biggest vent is on the right side, putting that on top helps heat escape through its natural tendency to rise.