High performance graphic cards have more work going on the most (all?) CPUs - so cooling is vital. Some cards have heatsinks on the memory modules as well. That said - unless you have overclocked the card, the supplied cooling setup should be adaquate.
Try playing the game with the side panel of the pc removed, whilst you've got the panel off, check the fan is still working on the graphics card.
Dust is a good insulator, so the more is coating the inside of your pc the hotter it can get, esp if it is on the cards. You can get air dusters (basically air in a can) which are good for removing stubborn dust/dirt from electrical circuit boards, from most pc shops at about £5
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