By my calculations 14.04GHz.
A 2500k would have to be clocked at 5.94GHz to hit 900.
My 2500k does about 1 CPUmark99 for every 6.6MHz. A P4 is 1 CPUmark99 for every 15.6MHz. Besides the increased numbers of cores the 2500k core is 2.36 times faster per clock cycle at this benchmark. Despite this being an old benchmark every core since the P4 has shown real world performance improvement and IPC improvement on this bench.
Here is a graph of my computer history with CPUmark99 results from each one of my systems.
http://www.hyperactivemusic.com/cpumark99.pdf
Without even taking increased speed of the memory buss, drive system, and all of the other improvements, my latest computer is 2500 times faster (when you count all four cores) than my first computer, a SX25.