I've been using a program called hdd thermometer for the last few weeks.As the program states my hd reaches to temps up to 52 degrees (Celsius) and is giving me a warning.

Do high temperatures lower the hard disk's life span ?
Do i need to buy a hd fan to lower the temperature ?
Should i buy a dual or single fan ?

High temperatures do lower the hard disk's life span. It is a good idea to cool the harddrive. This can be done with cooling bays, fans, heatpipes or whatever you want. If you got a good case, there'll probably be fan bays close the harddisk bays. Putting a fan in there (ususually 80mm) you can cool your harddrives adiquatily. Just make sure that the heat coming from the disks can leave your case as well, so make sure you have at least one outtake fan mounted in the back of your case (preferrably mounted in a higher position than the hdds are in, as heat goes up).