It does have truth to it, but often the chip is capable of speeds far higher than it's rated; this is simply marketing, low-clocked chips are sold cheaper and high-clocked chips for more money. Some goes for number of cores/other features.
On topic: IIRC Athlons have no L3 cache, that being a big difference. I've gathered the performance hit is around ~10% tops, but I haven't really been following the CPU situation as of late. Anyway, in any real-world apps the difference is negligible; you'd most likely only be able to actually tell the difference in heavy rendering and other crunching apps, in which case you'd better off looking at something like an i7.