Mine is around 43-45'c. But that is when I just boot it up. When I start to play a game then finish up. It will drop to around 48-50, it will take some time to drop till around 45'c.

I don't really know, but if you refer to other reviews on the web the idle temp should be lower than 50'c. How about you try to run your system outside the casing to rule out that the airflow might be the culprit. I'm using 2 reference GTX580s as well.

Then I guess it's the card then. 10MHz overclock won't make any differences at all.

what temp monitoring tool u used to check the GPU temp?
try using different tools & see.
u might wanna try re-applying TIM in which i believe would reduce the temp.

I would suggest you to try to RMA. But don't expect a good outcome. Based on my experience this kind of case is hardly will be entertained by the shop or distro. But you could try to push your luck, clearly 55'c is not the right idle temperature for the card especially in air-conditioned room. That's if the monitoring software that you are using is displaying the temperature correctly

If RMA fails, you could try as owikh suggested. But bear in mind the risk if you don't have any experiences doing this, perhaps you could try to find someone that have experience in stripping naked a GTX 580.

This is my current temps, with the casing's side panel opened using MSI Afterburner BETA

Should be okay. Mine is around that temp too when I'm not using my own fan profile

i also got the same asus reference gc & same clock as yours @kakicam
the temp also the same.. btw the gc i bought 2nd from lyn forumer..

GTX680 wooot woott 

Just try to setup your own fan speed profile accordance to your noise tolerance level for your GTX 580. It helps