I recently bought an ATI HD6870, replacing my old and very noisy and hot HD2900XT+.
As far as it seems it was an really good idea, as the new graphics is just awesome in performance and price (~180€).
By the way, I would recommend the Sapphire "Not-Reference" Model which comes with an special cooler - and is damn quiet.

And another thing: ATI brought out an Preview of the new 11.4 Driver called "Mjölnir 1" which does improve overall performance.
Its not stable yet, but maybe you want to check it out: http://support.amd.com/us/kbarticles/Pages/Catalyst114earlypreview.aspx
( Windows 7, Vista Only )