AMD Athlon 64 FX-57 Review
It's a bit more complicated than that, I'm afraid.
For example, the number of pipelines is important. For example, Athlons have nine (or did at one point, I haven't looked at this one specifically). 3 x86 decoders, 3 fp, and 3 int. A P4 has six. 1 x86 decoder, 2 fp, and 3 int.
What that means, is that an Athlon performs ~9 operations per cycle, or 9 * 2.8 Ghz = ~25.2 billion instructions per second, and the intel would do 6 * 3.8Ghz = ~22.8. Those are very, very rough estimates.
Of course, those numbers actually depend on what instructions are actually sent (if you send all integer instructions, the P4 would come out ahead, etc.)
There's more too. The amount of cache effects how often the pipes are used (more cache, less pipe usage, is less waiting, which is faster). The size of the pipes is important (you may be doing more per cycle, but the cycles take longer). How well they can estimate future instructions, etc. Lots and lots of other things too.
So basically, the Ghz of the processor doesn't mean a whole lot. There's a lot more to take into consideration.