The fastest a single core was reportedly taken was around 8.3GHz with an intel pentium prescott, if they are to be believed.
I have seen may people claiming that their 4 core cpu is running at "16GHz" because they have 4 cores each running at approximately 4GHz. This is not, however how speed rating work, and their cpus are infact running at only 4GHz.
Even with a multi-core processor, most programs are only able to use 1 core, so each core runs a different process (This is why if you look at the hardware monitor of a multi core pc under normal use, some cores are at idle while others can be running at 60% plus).
In addition to this, newer cpu's use something like intel's speedstep (similar technologies exist with ati and nvidia) technology to vary the clock speed up to the processor maximum to save power while at idle. For example an i5-2500k could be overclocked to 4.5GHz, butonly run at 1.6GHz due to speedstep reducing the multiplier to x16 with a buss speed of 100MHz)