Expreview published some benchmark figures of the upcoming Radeon HD 4670 and Radeon HD 4650, both cards are vased on the 55nm RV730 GPU. You can check it out over here.
700MHz/1000MHz(4650/4670), shader count is 320, the same with RV670. But memory interface is only 128bit, which sounds very low end…
In benchmarks 3DMark Vantage shows HD 4670 still can not easily catches up with HD 3850 512MB. But in actual games like HL2EP2 and COD4, the card performs very well, only a few FPS less than 3850.
The site claims the Radeon HD 4670 performs close to the Radeon HD 3850 but that's not entirely true, on most of the benchmarks there's a gap of at least 10-15% between these cards. It's quite a bit faster than the GeForce 9500 GT though.