I think I know why its only specced at 2Ghz.
Its because I suspect, both cores won't be working all the time (Like a Pentium 4 Hyper Threading does sort of), and so, with the two cores, you'll get a system performance of a 3800+ Intel CPU or better, but because it's dual core, you'll feel it being a bit faster because of it being able to run more programs at once, etc.
Also, because it is Dual core, AMD is able to drop the running GHZ (In AMD terms: 2.0Ghz), but still get the power of a 3800+ Intel CPU.
I'm not sure though I'm afraid if each core is calculated at 2.0Ghz, as I wouldn't like to speculate, but I can say, that it still equals a 3800+ if it does or not, and I suspect also that it just runs the same way as Hyper Threading does on a Pentium 4 EE, Though with 2 true cores.
So to conclude, it wouldn't be like having 2 CPU's, as in that case, performance would tend to double if Windows, and programs used them more.