Allegedly, Radeon HD 4870 X2 prototype 3 DMark Vantage may be at about X5500, and NVIDIA's flagship GeForce GTX 280 only X4800 points, a difference of nearly 15 percent
3DMark is the ideal 3D application where you can make the most out of a multi-GPU card, both because Futuremark has designed it to take full advantage of multi-GPU and because it if the first application where the video card drivers are optimised for. Traditionally SLI & Crossfire performed better in 3DMark than in games, so I expect the ATI 2-GPU design to be better favoured in 3DMark than in games