Today is the launch of the long anticipated AMD Radeon HD 4870 X2. This card features two RV770 graphics processors on a single PCB. The RV770 has been used on the HD 4850 and HD 4870 in the past. Another product announced today is the Radeon HD 4850 X2. The HD 4870 X2 uses GDDR5 memory, while the 4850 variant uses GDDR3. Compared to the single GPU designs, both X2 cards feature twice the available video memory per GPU. The HD 4870 X2 has 2 GB of GDDR5 - 1 GB per GPU. AMD is positioning their HD 4870 X2 as "the fastest graphics card on the planet" - which is reflected in the price of $549.
Sapphire's HD 4870 X2 follows the AMD reference design specifications to the letter, the only difference is the Sapphire sticker on the cooler.
One novel feature of the HD 4870 X2 is that it has an additional direct-GPU-to-GPU interconnect called CrossFire Sideport (XSP). The XSP offers an additional 5 GB/s interlink bandwidth between the GPUs but is not enabled at this time. Yes, you read correctly. The official reason why the XSP is disabled at this time is because that much bandwidth is not required with current applications and it will be enabled at some point in the future via driver update.
One major advantage of the XSP that I see is that transfers between the GPUs would have a lower latency. The Gen2 PCI-E bridge will certainly be fast, but it will take a short time to process the incoming data and send it out to the other GPU (<140 ns). With the XSP's point-to-point interlink this delay is eliminated. So my speculation is that AMD is working on using the XSP feature but the driver support simply doesn't work as intended at this time.
HD 4850 X2
HD 4870 X2
|Shader units||320||320||64||112||112||800||128||128||128||192||800||2x 800||240||2x 800|
|ROPs||16||16||16||16||16||16||16||16||24||28||16||2x 16||32||2x 16|
|GPU||RV670||RV670||G94||G92||G92||RV770||G92||G92||G80||GT200||RV770||2x RV770||GT200||2x RV770|
|Transistors||666M||666M||505M||754M||754M||956M||754M||754M||681M||1400M||956M||2x 956M||1400M||2x 956M|
|Memory Size||256 MB||512 MB||512 MB||512 MB||512 MB||512 MB||512 MB||512 MB||768 MB||896 MB||512 MB||2x 1024 MB||1024 MB||2x 1024 MB|
|Memory Bus Width||256 bit||256 bit||256 bit||256 bit||256 bit||256 bit||256 bit||256 bit||384 bit||448 bit||256 bit||2x 256 bit||512 bit||2x 256 bit|
|Core Clock||670 MHz||777 MHz||650 MHz||600 MHz||600 MHz||625 MHz||675 MHz||738 MHz||575 MHz||576 MHz||750 MHz||625 MHz||602 MHz||750 MHz|
|Memory Clock||828 MHz||1126 MHz||900 MHz||900 MHz||900 MHz||993 MHz||1100 MHz||1100 MHz||900 MHz||999 MHz||900 MHz||1000 MHz||1107 MHz||900 MHz|