It is based on the same RV770 GPU that is present on the HD 4850, too. The major difference is that the HD 4870 uses GDDR5 while the HD 4850 uses GDDR3. Certainly the biggest benefit of GDDR5 is that is offers twice the memory bandwidth than GDDR3 at the same clock.
As a result the HD 4870 has almost double the memory bandwidth available, even though its memory clock is slightly lower than the HD 4850. Another difference to the HD 4850 is that the HD 4870 uses a dual-slot cooling solution allowing AMD to increase the core clock by 125 MHz to 750 MHz.
Diamond's HD 4870 is fully based on the AMD reference design, but Diamond has already announced a special Black Edition card that allows much higher clock speeds.
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