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Also of significance is the inclusion of a whopping 1GB of frame buffer memory per chip for a total of 2GB for the card. ATI assures us that this extra RAM does bring a noticeable performance advantage, over using 512MB per chip, at the highest resolutions and AA settings. However, as we only have the 2GB card, this isn't something we can test directly.
As well as the flagship HD 4870 X2 with 2GB of RAM there will also be a 1GB (2 x 512MB) card as well as two HD 4850 X2 cards, also with 1GB and 2GB frame buffers. Just like their none-X2 namesakes the HD 4850 X2 and HD 4870 X2 are differentiated by the type of memory they use with the former employing slower GDDR3 and the latter using super fast GDDR5.
Now, the 2GB frame buffer issue has some interesting consequences when using these cards in 32-bit operating systems, as this massive amount of memory will gobble up more than half of your computer's address space, leaving less than 2GB for actually running programs. For the time being we've kept our testing to just 32-bit Vista but ideally you'd want to use a card like this with a 64-bit OS.
The only other notable features of the X2 all come in the way the two RV770 chips communicate. Just like the HD 3870 X2, the HD 4870 X2 uses a PCI-Express bridge chip to manage the majority of the communications but instead of being limited to PCI-Express 1.0, the new chip is PCI-Express 2.0 so bandwidth has doubled.
Not only that but ATI has also included a brand new feature called sideport. This is a dedicated connection between the two chips that provides an additional bi-directional 5GB/s of bandwidth. The result is a card that should never be left wanting when it comes to inter-chip communication.
Oddly, though, ATI hasn't actually enabled sideport. We were told this is because it's simply not needed yet and, in fact, may never be needed, so rather than dedicate driver development time to getting it working, ATI has left it unused.