If you've been reading up on ATI's soon to come release of Crossfire, you may have heard many shouts from the rumor mill concerning that Crossfire may have, including a solid cap of 1600x1200x60Hz, due to the Silicon Image receiver being used. In all reality, that's mostly bunk. The article discusses the hardware potential for Crossfire cards. One of the most interesting things is that the standard being cited says that the limitation is partly due to the cable length being 1 Meter in length, whereas the cable linking 2 video cards together is going to be vastly shorter than that. Higher frequencies may still be possible, indeed:

”The receiver can handle 1600x1200 @ 60 Hz, which means that it can receive 60 frames of 1600x1200 pixel content per second. When used with the output of the master card, this can give a theoretical maximum of 120 fps at 1600x1200 resolution when using alternating frames.”
Crossfire is not yet even in our hands yet so it's difficult to say exactly what it can and cannot do, but it's safe to assume that ATI has planned it to be a viable contender in the multi-GPU arena. It's a competition market, after all.