The Pentium M 1.6 up to 2.0 (and the Pentium M 1.5 if it is labeled a 715) is the new Dothan chip line for the Pentium M. This is a new manufacturing process and they were able to make the chip itself smaller so they added 2mb L2 system cache instead of the 1mb on the 1.5 Banias (705) and lower Pentium M's.
If you are ordering custom I would highly recommend that you get the Dothan Pentium M. The 1.6 and 1.7 are fine, the 1.8 and 2.0 are more pricey and only slightly faster.
I have a Pentium M 1.4 Banias but I bought it as a prebuilt Compaq X1000 in late May 2004 (before the new chips were released).
You will notice a difference with the increased cache when surfing the web, etc. as frequently accessed web pages are held in the faster cache rather than system RAM or the hard drive.
You will also highly benefit when running programs by getting a 5400 rpm or 7200 rpm hard drive (most desktops have this speed already) rather than the slower 4200 rpm.
Note that in prebuilts the 1.7 Dothan laptops being sold retail (like the Toshiba M35S456 and HPZT3380US) are fully loaded with a DVD burner, etc so it makes them nearly 500.00 more than a Banias Pentium M system with a CD-RW/DVD ROM,etc.
But if ordering custom you can just get the 1.6 Pentium M Dothan, get the faster 5400 hard drive at least, and get 64 mb of dedicated video.
It is smart to just order 256mb of memory (1 slot) and add 512mb yourself later for at or under $100.
OK, so i'm a soon-to-be college student, and I'd like some advice on the laptop I'll be getting. So far I've looked at the IBM R51, the Dell 8600, the hp zt3000, and the compaq x1000.
I was able to get an employee discount for the IBM r51, so that it totaled about $1440 for a Pentium M 1.6 GHz, 256 MB RAM (which i plan on upgrading in the future), CDRW/DVD, 60 GB HD, and the Intel 2200 Wireless.
My only concern is that the processor has a 1MB L2 cache, whereas the other laptops I've looked at have 2MB L2 cache for the same processor speed. Is there a huge difference between the two caches? I really like the Thinkpad, but do you think I can get a better deal with the other laptops?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic