Opinions vary, but to be honest, I don't really find it to be worth the effort. The way Windows is set up to work means you lose most of the benefits you'd get from using partitions.
Programs must install at least some part of themselves on the same drive/partition as Windows, so if you ever had to reinstall the OS, programs would need to be reinstalled as well.
There is something to be said for a partition that only stores documents and downloads. That can be useful, especially if you just want to point a backup program at it. Nothing else is really worth the effort if you ask me.
Also remember that it will take some time for computer makers to customize the Vista SP1 package to work on their systems, and even longer before they start shipping systems with it installed. The service pack Microsoft releases will be for the poor souls who actually bought Vista separately, and didn't get it with their computer. It may cause problems if you try and install it on Dell, HP, Compaq, Gateway, etc, system. So, I'd plan on getting that new system 3-6 months AFTER Vista SP1 is released if you don't want to install it yourself, and maybe 1-2 months if you don't mind installing SP1 yourself. Maybe even ask yourself how much you really need a new system, because the little bit I've used Vista makes me loathe it, and I'm usually the type who embraces change. The only compelling thing about Vista is the new Aero interface, which is a massive resource hog and a very poor imitation of Apple's Mac OS X Expose feature to boot. Otherwise it's just XP with a bunch of options moved about randomly, and about 3X the system requirements. If it were me, I'd just save my money and ride it out with XP until Vista's successor (assuming it will be any better), give Linux a shot, or buy a Mac instead.
After SP1 for Vista is released, I plan on buying a new computer. I have read that it is a good idea to put the operating system on its own partition to make it easier to image and reinstall. If this is so, what size should the partition be. Also, is it a good practice to have another partition only for software (other than the OS).
Thanks for any suggestions and advice.