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For a number of reasons we've had mixed opinions about graphics card manufacturers making cards that use two graphics chips on one board. For a start, they tend to be expensive, which is understandable considering the extra hardware but still lamentable. Then there's driver/game support that can, if not up to scratch, mean some games simply don't work or don't see any benefit from the second chip. Finally, there's the fact that even when everything's working fine, the performance increase from the second chip can be limited. It's for these reasons that ATI's last attempt at gaining our top graphics card recommendation, in the shape of the HD 3870X2, fell just short of the mark, receiving a good but not quite there, 8/10.
So when nVidia launched its competing dual-chip card, the 9800 GX2, we had that same sinking feeling in our bellies. That said, the 9800 GX2 does combine two very fast cards into one package that, unlike conventional SLI configurations, doesn't require an SLI motherboard and, at least theoretically, has the potential to be the fastest single graphics card on the planet.
Just as with its last attempt at a dual-chip card, in the form of the GeForce 7950 GX2, for the 9800 GX2 nVidia has gone with the arguably crude route of just bolting two whole cards together in one package - as opposed to the HD 3870X2, for which ATI managed to fit two chips on a single PCB. Not that this resulted in the HD 3870X2 being any smaller than the GeForce 9800 GX2 but it did allow the former to have a better cooling solution (more about that later).
Where nVidia has improved on its last attempt is by refining the package so that both cards are now completely contained inside a metal shroud, which keeps all the delicate electronics protected from our grubby mitts. The two cards then communicate over an internal ribbon cable that's linked to a dedicated PCI-Express bridge chip on one of the PCBs. This enables the SLI communication that's needed to power all this dual-chip madness to be performed completely 'on card'. The massive advantage here being you don't need an SLI capable motherboard.
The two cards have also been manufactured in mirror image so that both chips face inwards, which enables nVidia to use a single large fan at the back of the card to suck air in, blow it across both GPUs, and exhaust it out the back. Unfortunately, because of the way the card's outputs are configured the actual exhaust on the back plate is tiny, which is going to restrict airflow considerably, leading to potential overheating. Indeed our friends over at bit-tech found there were some other heat issues with the card that resulted in the north bridge chip of certain motherboards overheating. Basically, be aware that you'll need a very well ventilated case to successfully use this card.