Fyi, the term "Centrino" is just Intel's fancy name for the combo/bundle of: M processor, 855 chipset, and wi-fi card. Take any of those away, and it is no longer a "Centrino".
The Intel wi-fi card is worthless IMO though, so if that is a concern, seek out another option. For example, my mobile PC is a Dell Latitude D600 that was originally marketed as a Centrino, but since I ordered it with Dell's internal (a/b/g) wi-fi card, it doesn't carry the "C" badge. With a gig of memory (probably overkill for what I use it for), mine flies. The weak-link is now my slow HD.
I'm not sure I would go M on a desktop though. The new Pentium "X" (dual core) processor might be worth checking out. If it isn't out yet, it should be available soon.
Here's some more info:
http://www.intel.com/products/processor/
http://www.intel.com/products/processor/pentiumm/index.htm
http://www.intel.com/products/processor_number/