core clock pertains to the architecture that it is built on. we had 3.0ghz dual core pentium cpus many many years ago now but they will not hold a candle to a modern, say, sandy/ivy/haswell dual core clocked at 3.0ghz. not only is performance per cycle faster, the new processors have different/newer instruction sets that are more efficient and they are also more efficient at the older instruction sets.
if your comparing like processors from the same family, then the quad core is going to be faster until you jump to a program/app that can take advantage of more than 4 threads. if your comparing amd to intel, 4 intel cores running the same clock speed are going to be noticeably faster than 4 amd cores running the same core speed.
what two cpu's are you trying to compare exactly, an fx6300 and a x4-760k/a10-5800k? the fx6300 is going to be faster all around as it based on higher power high peformance bulldozer architecture while the richland architecture is meant for lower power efficient performance. richland is also based on an older fm2 socket with accompanying chipset while the fx6300 is on a much newer and more features based am3+ socket/chipset design.
if your talking about an fx4300, well its the same as an fx6300, just minus 2 core and slightly faster on the remaining cores and it will be slightly faster in light apps but keep in mind both the fx4300 and fx6300 are unlocked and they both can be easily overclocked to a modest 4.2ghz with a simple cheap cpu cooler and after that is said and done, the fx6300 is the way to go.