Sapphire should be the cheapest followed by PowerColor, but don't get Sapphire if the card is not based on a reference design from AMD (it will save you some possible headaches). MSI is good.
Sorry for not mentioning the system. Is the present PSU-CM exreme power plus 600 sufficient for running R6870 GPU, or do I have to replace it with some higher model. Currently he is running a 2300 in a Intel H67 mobo.