+1 on cleaning up the card (and maybe your case). I'm not 100% sure that its even a temperature issue, but there are 2 things you can do to prove that it is. You could change the fan profile for the card, first to 25% -- and see if it artifacts all the time or sooner. If you prefer not risking this, then the other option is to change the fan speed higher than 50%, maybe 75% and see if it takes longer to artifact or mess up. The second thing you can do is to play another game. As crazy as it sounds, it could simply be an incompatibility with Medal of Honor, although unlikely.
If you consider getting a new card, I would advise to wait for the 69xx series release this month because you might be able to get a more powerful card for the current price of what you'd pay for high end right now (e.g. the ATI cards might be more powerful than the GTX480 for the same price, OR the GTX480's price drops after the release - so you can get it cheaper). This will all happen in about 1-2 weeks, so its not much to wait if you can.