I too was laboring between these two machines. I went with the zv6000 because I wanted the ''speed'' of the Athlon chip, but as you may or may not know, this chip was ''handicapped'' by HP's choice of motherboard.
However, I still detect a difference in speed between the two chips when tested side by side - the Athlon 64 being the faster of the two. My chip is a 3800+ and I believe it is more of a power hog than the 3500+. If I could do it again, I'd get the 3500+, save me some $$ and get longer battery life.
Here are some other details that might help you decide. I have listed some plusses and minuses for each machine based on the way I use my notebook so this may not help you at all.
1. I use my notebook for photo editing and have done videoediting on the zv6000. Although slower than my PC, it did the job well. My friends Pentium M machine was even slower than my zv6000 by about 30%. But if you do not do this type of heavy work, then you might not feel the difference in their speeds.
2. Battery life in the zv6000 is not that great. My upgraded 12 cell battery lasts about 1.75 hour when watching a movie and I do not find that it lasts longer when doing routine tasks. I have wireless on most of the time and I run NAV in the background so I'm guess that uses up a bit of energy. At this point, I just use it with an AC cord unless I must use the battery. My friend's Pentium machine has a noticeably longer battery life - however, her notebook has only a 14'' LCD screen so this isn't really a fair comparison.
3. My Brightview screen is quite nice for viewing prerecorded movies and photos and if you order this machine consider this option. There can a problem with glare from lighting that is behind you, but the brightness and color rendering make it worth it in my opinion. I just reposition myself or the screen when glare is an issue. The integrated video on the dv4000 is not very good, so if you buy that machine, you might wish to pay for the upgraded video card (128MB ATI MOBILITY(TM) RADEON(R) X700). It is expensive at $149. I ordered the much less expensive upgraded video option on my zv6000 - it is NOT as good as the upgrade on the dv4000 and if you play graphic intensive games alot, I think this might affect your choice. I NEVER game, so this was not an issue for me at all.
3. The audio sound on my zv6000 is actually better than all of the notebooks I heard in the stores (excepting the 17'' media notebooks) and I thought the sound of the zv6000 was substantially better than that of the dv4000 when I tested them side by side in Office Depot. Using Media Player's equalization option, I am able to get a nice warm sound when listening to classical and a fairly full sound when listening to rock - but DON'T expect a lot of volume. When playing movies, use the included WinDVD (not the Home Theater suite) since it allows you to adjust brightness, contrast, color balance and also features DSP for the audio. MUCH BETTER THAN Windows Media Player.
4. dv4000 has a quick launch option for playing movies which DOES NOT require booting up Windows, so it kind of doubles as a portable DVD player. I liked that idea alot and that was my main attraction to the machine. However, when I checked it out at Office Depot, I just didn't think it felt as well made as the zv6000 and I didn't like its sound or picture nearly as well. The upgraded video option will probably make a big difference in the picture, but there is no fix for the sound except to use external speakers.
5. HP really rips the high end customer off with its excessive prices for RAM so another advantage of the dv4000 is that it can be ordered two different ways with one memory bank filled. You can order the single 512 DDR2SDRAM or 1024 DDR2SDRAM and then purchase an additional module of 512 or 1024 later - max 2GB of RAM! I am pretty sure you can mix RAM size on this machine. Its a very easy upgrade to do on your own. Just be sure you exert care when buying the RAM to be sure its compatible.
You can order the zv6000 with a single 512DDR2SDRAM but there is no option for a single 1024. Since I think that the RAM must match in size on this particular machine, you can only put in a 1024 maximum? But don't take my word on this - call HP sales for some information. Also, if you order a custom machine, be sure to consider a 5400rpm hard drive.
If you do not plan to do much in the line of music, photos or video with this machine, then I think you might like the dv4000 very well. It is a smaller lighter machine with a longer battery life. Its chip is not 64 bit, so you won't be able to make the jump to the upcoming 64 bit Windows OS, but it is quite possible that this OS will require so many more resources than our laptops contain that it will be a moot point. Thus, I'm not sure I'd let that make up my mind for me.
Finally, if you are still tormented by indecision and if you are a Costco member, try looking at their site for pricing. They aren't always the cheapest, and you will pay tax and delivery, but you can configure your own there AND if you have 6 months to return it NO QUESTIONS ASKED. This is a deal unmatched by ANY retailer out there. I bought at HP direct using my teacher's discount, free shipping, no sales tax posted, so my price was a bit better than Costco could offer. You will have 30 days to return it to HP shopping - no questions asked, but the 6 month option is very very consoling!
I really like my zv6000. It is beautiful to look at, handles and operates well, is well featured and performs to my satisfaction.
Hope this helped.