There really is no equivalent to the FX-55 or 53. They are the top of the line. Intel suks big time, they blow chunks! The only P4 that can even remotely keep up is the P4 3.62GHx Extreme Edition, which is 250 dollars more then the FX-55............. Screw Intel and their equivalents.... They are dumb. BTW, I use both types of chips, and love AMD far more. My SLi blew up!!!
So now I am using this P4 system, and I really see some speed differences. The only good Intel chip is the Pen M chip, which is very good and low energy, but AMD64 Mobiles are almost as good, plus they are 64bit.
Clock speed algorithms are different, so an equivielent to the AMD FX-53 would be around 4.0GHz or so, give or take 200MHz. But the FX is 64bit, so later, it will be useful, and P4 will be dead.