AMD's ATI launched their latest attempt to conqueror the important mainstream segment of the graphics card market.
These new competitors both contain a 55nm 320-core TeraScale GPU, which is a modified (read dumbed down) version of the RV770 chip found in their big brothers of the HD4800 line. Basically, it is a lower-spec chip that contains the same features. So in human words: it can do the same tricks, just slower.
They both have 512MB of ram. The $60 costing 4650 has DDR2 whilst the 4670, which is $20 more expensive, gets DDR3. Power consumption is quite OK for such a type of cards at 60 and 50 watt under full load.

As usual, when AMD launches a product they lift the NDA's so all the big sites can put their product reviews online. Guru3D one of my favourite hardware sites had this to say about the HD4670:The card thrashed its direct competitor in price range, the GeForce 9500 GT and is getting near the GeForce 9600 zone for a much lower price.
Slap a passive cooler on this puppy and you've got yourself a great HTPC card with all its Blu-Ray features like image optimization and and 8-channel sound throughput.

In other news: