The HPZd7000 is a desktop replacement in every sense of the word. If it will stay on a desktop or table in one building and you want the 17" LCD and Pentium 4M power for gaming and all other facets of a hot running, low battery life notebook (in other words you will leave it in place and on A/C the majority of the time) then these are just like a desktop with a 17" LCD flatpanel -- except all in one piece and it can be moved around more easily and you don't have wires going from the the LCD to the desktop. But they will get 2 hours or even less on a battery charge due to the huge LCD screen and hot running Pentium 4M. Toshiba and Sony also make full-blown 17" notebooks if you want to go down this avenue. But the notebook alone (don't even talk about the huge A/C power supply) is close to 10 pounds buy itself.
If you still want to have a large enough LCD to take advantage of DVD's (especially in widescreen), enhanced viewing area on spreadsheets (wider screen gives more data area on both sides) and still have reasonable portability then the other choice you made, the Dell 8600 is a better choice (Pentium M CPU and 15.4" widescreen).
If you want to stick with the Dell there are a lot of students that buy that. However, its drawbacks are that its case was not designed for the Pentium M which requires thickness due to the fact it runs much cooler. So the Dell still weighs 6.9 pounds. And the keyboard is not quite as good as IBM or the models below.
If you want a 8600 or equivalent I highly recommend the Toshiba M35 series (prebuilt example is M35S456 for $1999 retail) with the 1.7 Dothan Pentium M processor. This machine has everything and weighs 6.2 pounds. But its price is higher as it has a DVD burner that adds $200 to the price.
I have a Compaq X1000 series which is nearly identical in specifications to the Toshiba but it weighs 6.5 pounds. It can be completely custom ordered online and it is identical to the HPZT3000 except for appearance (HP is all black inside except for touchpad, Compaq is half silver and half black). By ordering custom, not only can you get a $100 rebate, qualified education rebate % discount, etc. but you can order it exactly as you want. For example, the 1.6 Pentium M is the same as the 1.7 (Dothan, 2mb L2 system cache) but it is $125 less so most people ordering custom get that. You want at least 512mb memory but you can order 256mb from the manufacturer if ordering custom and get Crucial or other memory yourself for much less (or add 512mb more for about the price of 256mb more). You can upgrade the video to 64mb for $50 more than 32mb, you can order a faster rpm hard drive], etc.
Toshiba can also be ordered custom as an M30 online.
Go to X1000forums.com to see how low you can get the pricing if you are interested in either the X1000/ZT3000 or HP business nx7000 notebooks.
Or, see notebookforums.com to read up on Dell and Toshiba but again they don't have forum for just this particular Pentium M notebook like Compaq/HP has.
Dell also is custom order of course so follow the same rules as above if you prefer the 8600.
Remember, video memory cannot be upgraded so choose that very carefully (and make sure you get a notebook with dedicated video RAM (ATI or Nvidia video card) not integrated video -- which draws off your system RAM.
And I would get no lower than the 715 1.5 Pentium M (HP and Dell don't offer this -- they offer the older Banias 705) or Pentium M 1.6 725 cpu.
As a benchmark, Pentium M's are comparable to a Pentium M cpu up to 1.5 times their mhz speed.