Us usual, Apple reduced the clock speed of the Radeon HD 4670 used in the new iMac. This fact was identified by Netkas when he was looking into the bios. The default frequency from the reference card from AMD is 750 MHz for the GPU and 800 MHz for the GDDR3. Apple modified the frequency to obtain 680 MHz for the GPU and 790 MHz for the GDDR3. If the difference is not huge for the VRAM, the drop in clock speed is more than 10% for the GPU, the Radeon HD 4670 is then offering performance levels between a true 4670 and it sister the 4650 with its 600 MHz GPU. Last but not least, the las beta version of Mac OS X 10.6.2 offers drivers for the Radeon HD 46x0, 45x0 et 43x0, so one can expect to find such GPU in future MacBook Pro that could be released ahead of expectations.
