I'm considering a purchase of a Dell laptop.  The choices  are a 
IntelŪ PentiumŪ  M Processor at 1.4 vs a 2.8 Intel PentiumŪ 4
Processor at around the same price.  Will I notice a difference in
performance between the two? Any other comments?

blue_bna-ga:

Thank you for your Question about the performances differences between
a 1.4GHz Pentium M processor, and a 2.8GHz Mobile Pentium 4 processor.

The answer will depend on your actual usage patterns, but generally,
the mix of processing capacity and power efficiency of the Pentium M
chip makes it superior to the Mobile Pentium 4 chip.

Here is one article published by UPenn's Computing group that
discusses the differences between the two processor families:

http://www.upenn.edu/computing/provider/docs/centrinoprovider.html

Key differences to note are the larger L2 cache in the Pentium M
design, and the use of the 855 chipset (which delivers the integrated
power rationing capabilities which makes the 'Centrino' concept of
long battery life a reality).

Since the graph on that page is a qualitative one, let's turn to
another study, this one by Intel itself. This study is referenced in
this article:

---
http://www.theinquirer.net/?article=10964

"THE PENTIUM M appears to be such a fantastic chip that it knocks the
socks off the best Intel can offer with Pentium 4 technology and AMD
with an XP 3000+."
---

The Intel study can be found at:

http://www.intel.com/products/mobiletechnology/docs/performance_brief.pdf?iid=ipp_perf+perf_brief

If you scroll to page 15, you will see that the 1.6GHz Pentium M
processor (and the 855 chipset) is vastly superior to a 'faster'
2.4GHz Mobile Pentium 4 in both processing and battery life. This gap
will be a lot closer when comparing a slower Pentium M with a faster
Mobile Pentium 4, but the Pentium M will still be highly competitive.

My own experience using laptops with Mobile Pentium 4's and others
with Pentium M's is that the Mobile Pentium 4 runs hotter and is a
major power hog, while the Pentium M-based designs tend to be
extremely efficient and surprisingly fast. I'd recommend NOT
attempting to use a Mobile Pentium 4-equipped laptop on your lap, you
might get a nasty burn.

So, even though on first glance it might appear that the Pentium 4 (at
2.8GHz) should be the 'faster' processor, the reality is that the
'slower' Pentium M (at 1.4GHz) will be able to process almost as fast,
and will definitely do it for a lot longer while running on batteries
than the Mobile Pentium 4.

I hope this information helps!

aht-ga
Google Answers Researcher

Well done,thank you.  Told me just what I needed to know.