Driver for ATI radeon HD 4300/4500 for Leopard 10.5.6 pls?

Hi i installed Leopard 10.5.5 and updated to 10.5.6.I am using ATI radeon HD 4300/4500 ..But i cannot change the resolution.I am using samsung lcd monitor which has the resolution of 1600x900.But with mac it is already set to 1024x768 and no other resolution.Pls help me..with a driver..pls dnt suggest an update or gfx card..Thanks in advanc..