755 is more secure than 777 - 777 means anyone could read/write/execute the file. 755 means only the owner can read/write/execute the file while the group can only read and execute and the world also can only read and execute. See http://codex.wordpress.org/Changing_File_Permissions for more details.
@mdavarynejad - are you still seeing a internal server error?
@usefulhack - please start your own topic. While I don't doubt you are experiencing a similar problem, it is extremely unlikely that you are using the same hosts, on the same servers, with the same themes and plugins as the Original Poster! Think of these forums as help tickets and please create your own.