Retail 2600X2 cards will be available in Q4 of in October to November time but the Quad driver might not be ready at the same time. Quad Crossfire is possible and AMD did solve the driver overhead and it is working on the Quad crossfire support as we speak.
At least theoretically two Radeon HD 2600X2 cards rendering together can go after Nvidia's high end cards and definitely two RV670 on a single PCB rendering in Quad crossfire might even beat the fastest of Nvidia.
We are sure that Nvidia will revive its Quad SLI works as we can remember that two 7950GX2 cards were working in quad setups but they didn't get satisfactory scores. It was the laughing stock of Nvidia so they better make it much better this time.