Doing anything to the stock heatsink other than cleaning dust off it will void warranty.
You can purchase an aftermarket GPU cooler, but again voids warranty.
Have you created a custom fan profile in MSI afterburner? The stock fan settings are quite mild. If you want lower temps you can set the fan to a higher RPM for given temp and it can help keep the temps down.
What temps are you seeing? If the card is stable I wouldn't worry about temps unless it is noisy.
My 7970 was too hot it would crash if I overclocked it on stock heatsink, I changed it for an Accelero 7970 cooler but was a pain to install. It dropped temps from 83C under load to 56C MAX - while being silent. But now if my card dies, £300 down the drain because I can't send it back.
edit: OK, looking at your card, I can see why it would get hot, but I wouldn't worry about it. It has a very small heatsink and fan, there isn't much you can do about that getting hot really other than putting a bigger heatsink on it or putting the fan up. AS5 will only aid the transfer of heat from the GPU to the heatsink, but if the heatsink can't cope it won't make much difference.
http://www.msi.com/pic/product/five_pictures1_2385_20110418153234.jpg Is that the one?
You could try something like an Antec Spot Cool
http://www.overclockers.co.uk/showproduct.php?prodid=SY-000-AN or
http://www.overclockers.co.uk/showproduct.php?prodid=HS-007-AN&groupid=701&catid=2330&subcat=399. But tbh if you want a cooler graphics card that is £12 towards a new one and nearly 1/2 the price of the card itself.
