I have an Asus GT8800 card. I live in a hot weather country, and with summer coming, the temperature of the GPU is around 55C while idle. The fan keeps switching speed from low to medium speeds, and is quite noisy (I have the smaller fan size).
 
Is there anything I can do to improve cooling of this card?

If your a DIY kinda guy , install a after market GPU cooler , if not , maybe clean up the cabling inside your case and see what your case fans are doing(ie what direction they are blowing , and general location and if they are still spinning )

Get RivaTuner and set direct fan control... set it to 50% for example so it constantly runs at this level. Will keep it quiet and provide enough cooling. Don't need to be too scared of temps on these - they are fine running hot and a 90+ degree load temp is not uncommon.

I'm the DIY kind :-)  
With the "after market" hint I was able to figure out there are coolers for these cards.  
I ordered an Accelero S1 and will install it with a 120mm fan.
I'm sure my noise issue will go away.
Thanks

I got the same cooler as well , i must say i was impressed how well it cooled my video card , my temps dropped 20 degrees lol .But i would make sure you have some cross flow across the card to wick away the heat or buy the fan module , also follow the instructions about the ramsinks , i used rubbing alchohol and a eraser to make sure the ram had no residue and the ramsinks are solid.

I plan to install a 120mm fan at low speed over the S1.
Thanks for the tip regarding the ramsinks.

That should do the trick. a 120mm fan is going to blow much more air per revolution than a stock fan. You can probably set it to low and forget about it.