I’m considering the newest Mac OSX upgrade for a couple of our newer Imacs. If you are considering the same, you should note that Leopard is a bitter cat; it requires hardware with a bit more umphh than it’s predecessor:

RAM:
Tiger: 256 MB
Leopard: 512 MB

Disk Space
Tiger: 3 GB
Leopard: 9 GB

Processor:
Leopard is not supported by G3s, and G4s with processors below 867

Here’s a range of viewpoints I’ve heard to date:
“I’ll try updating my personal machines, but won’t consider it for production machines!”
“It’s really cool, new Leopard apps are already available, and I upgraded the day it came out.”
“I never upgrade my computers. I’ll get Leopard when I break down and buy a new Mac.”

For us it’s still wait and see. My advice for most users: if your system is already taxed to the max, this upgrade may slow down your work. Still, an increasing number of applications are Leopard only. I’d upgrade only machines that require one of these new applications. And you should forget about dual-booting these into concurrent Windows/Mac operating systems while running Leopard, unless you’ve got loads of RAM.

From Apple: Leopard System Requirements

As far as Windows Vista, I’ve not felt the need to upgrade any of our XP systems, nor do I know any good reason to make the switch. On the other hand, I’ve heard plenty of reasons to avoid it! Here’s a video from an editor at PC World that advises against it for most users. Check it out.

My decision (and advice to you as well) don’t upgrade to Vista unless you have a specific reason. Just wait and get it pre-installed next time you buy a system. If you are a Vista fan and think I’m full of beans, I’d love your constructive comments!