I had a severe low FPS issue for a short period of time when I had my 4870 512. With player chat enabled and in use (grouping with my brother in law) my game was crippled. Before that I could essentially run it full banana with medium shadows, 4xAF, no AA, 1920x1080 and was seeing 120+FPS. During my chat enabled settings, I was dropping to 15-25 fps.
I never really investigated, instead opted to chat through text. From some very minor web sleuthing, I have heard that Gigabyte motherboards boards onboard sound/Mic + 48xx video cards with HDMI connections are being blamed by some. My brother in law has a much more dated system (Dual Core AMD 4400+ with 8500GT) in his Dell, and he suffers zero loss of FPS during chat enabled sessions.
Nothing concrete, but might be something to check out.