The Inquirer writes both the current and the next generation of ATI's graphics card will be limited to a refresh rate of only 60Hz at a resolution of 1600 x 1200 pixels when you use CrossFire. The reason for this is because the CrossFire design uses a SiI chip which isn't fast enough.
ATI simply didn’t have the time to include a composition engine in its chip and that is exactly what Nvidia did. G70 and NV40 were designed with SLI in mind, and R520 and especially R420 generation weren't. So ATI had to find an answer and had to make a compromise and use the SiI chip. PCIe bus communication is simply not enough and you have to connect two cards and put two pictures together.
Probably we'll have to wait until the R6xx generation in 2006 before ATI will have a solution for this problem. In the meantime people will have to live with the fact that the refresh rates of ATI's CrossFire suck at resolutions above 1280x1024. But anyway, most people don't game at those resolutions.