the Athlon 64's are capable of running 64-bit applications, when they do, they should theoretically double in performance
also, the Semprons and Athlon 64's can process every instruction that comes in 32-bit, just like 2 people speaking english, one person can speak english better then the other
when people look at their CPU's, too often do people judge a CPU by it's frequency
by doing so, they are, well... right and wrong, but mostly wrong
they are normally right when judging CPU's of the same architecture, like a 2.8GHZ P4 beats a 2.4GHZ P4, although sometimes they are still wrong when comparing higher frequencies in CPU's of the same architecture; it has been known that some overclocks have resulted in a CPU performing worse than at lower frequencies, I think this is more common on Athlon XP's
the frequency doesn't tell you how many instructions a CPU can process at a time, which is why the measurement of 'flops' has been made - you might have heard of megaflops or gigaflops before, this is the number of instructions a CPU processes in 1 second