Dual cards such as the nvidia 7900GTX in SLI, or the ATI X1900XT in crossfire, are meant for gaming. They can split up the screen so they dont have to do as much work, resulting in a much higher framerate. Gameplay settings can be turned up such as resolution, lighting, antialaising, filters, ect...
Dual graphics cards, as these guys have said, is just running to two graphics card in tandum. nVidia use SLI and ATI use Crossfire, though SLI is a bit more tried and tested. In my opinion, there isn't really a call for it at the moment because there are only a handful of games that support it. Benchmarks from Digit-life (Call of Duty 2 and FEAR, show the framerate is bumped by about 30 - 40%. Personally, I don't think you'll need dual cards until further down the line, most modern cards can pump out enogh frames a second to keep you happy, SLI and Crossfire are just gravy.
yea I agree with mikeybob. A single 7800GTX, or even GT, give more than enough fps. Hell, I'm happy with my x800XL which is a much lower card than than the 7800GTX.
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Dual graphics card as in SLI dual cards? Or as in a 7950 2 cards in 1 PCIE slot?
Dual cards are used in SLI and Crossfire on rigs. the 7950's are used in quad SLI (4xVideo cards).