+1 @Mactronix. Faced with this exact same decision myself, last night I ordered a HIS ICEQ4 4850: http://www.newegg.com/Product/Product.aspx?Item=N82E16814161244. I considered a 4870, but didn't think I really need the added power, and don't want to CONSUME the added power, which is also why I didn't seriously consider nVidia around the same level, as they drink more juice.
I installed a HIS ICEQ card in my father's PC last year, and noticed how quiet that cooler was, and I also would prefer the heat to be exhausted, so I chose the HIS IceQ4 model of 4850 for myself.