I'm trying to decide between the two. The price is the same, give or take 10$. The 4670 is newer and has some perks but, the 3870 seems to be the strong card.
I was reading this review to get an idea. They said so many good things about teh 4670 that I was pretty confident it was a great buy. However, I looked at the specs on the 3870 and, well, the numbers are much higher.
I am very happy with my HIS IceQ3 Radeon HD 3870 card the only problem I had was that when I first bought the card there was a problem with the drivers in Linux OS but I cured that fast. on my Windows box it runs great I can Run any game it runs cool and compared to my old x800 card I had this thing is silent under loads