It's not worth it. Very few things on your computer are processor-limited; most often the processor has to wait on other components, like the hard drive (most common, especially if you don't have enough RAM), memory bandwidth, video card (in games, although the XPS2's video card is beefy enough that it shouldn't matter), and the like. Even for those few things that are processor-limited, $300 for 6% performance gain doesn't seem worth it.
Processor speeds can only be compared between similar architectures. It so happens that Intel's Pentium 4 architecture is the most inefficient per clock cycle, so other architectures (like the Pentium M in the XPS2) can be just as fast at much lower clock rates.
A general rule is that you can multiply the clockspeed of a Pentium M or Athlon 64 by between 1.5 and 1.7 to get the equivalent clockspeed of a Pentium 4. For instance, my Athlon 64 goes at 2.2GHz; it's roughly equivalent to between a 3.3 and 3.7 GHz P4, depending on application.
Pentium 4's tend to do better in: video and sound encoding, numbercrunching that involves repeated operations on a small set of data (like code cracking). They have the worst power efficiency of the three, and are rarely used any more.
Pentium M's tend to do better in: office applications. They have the best power efficiency of the three main notebook processors, but are the most expensive.
Athlon 64's tend to do better in: gaming, scientific computing, numbercrunching that involves large sets of data. They have moderate power efficiency and are relatively inexpensive.