Is MacBook Pro warming during gaming normal? I have a 2010 MacBook Pro MC371 (Core i5, GeForce 320M, 4Gb RAM). 

When I launch a game either new one (i.e. Starcraft 2) or rather old (Unreal Tournament 2004, bootcamped) my MacBook gets rather warm, even hot in the top left corner, around the place where magsafe charger is plugged in. The temperature is more than 40C. The fact that confuses me is that this warming occures even when I play old games.

Is that OK?
Yes. That's normal. As for "okay" -- it depends. The heat is coming from the extra graphic activity and CPU activity you're asking your machine to perform when playing the games. The MBPro is vented at the back, along the hinge between the screen and the keyboard, you want to make sure that area is free of obstructions and the hot air can leave escape unimpeded. Long-term exposure to high operating temperatures can certainly be detrimental to the components in your machine. But for the most part, if your machine's venting is unobstructed, it should be able to cool itself adequately.