First, MHZ does NOT matter in regards how "fast" a CPU is. Intel has more MHZ for years now and still, the AMD ones score all in all better (particularly in gaming).
The thing with the AMD cpus is the architechture which is far superiour than the Intel ones. Thus allows AMD to clock their CPUs lower which gives a better cooling.
In regard of your decision, the AMD64 looks much more promising than the 3.2ghz Prescott (see chart below, taken from Toms Hardware). You will notice even the fastest AMD cpus have a clockspeed of 2.4 ghz. It is kinda like Diesel-Cars. We all know they have much more power with lesser HPs than petrol ones, because the whole ignition system is different.
Of course, you have to get a new Motherboard if you go with AMD which will cost you a bit. But look at the chart and you will see for yourself.
