Put down the credit card. Turn away from the Apple Store. Of course you want to jump on the latest Apple bandwagon and upgrade to Mac OS X 10.5, better known as Leopard.

It has been almost two and a half years since its predecessor, Version 10.4 Tiger arrived, and the new operating system offers a slew of changes, including a revamped user interface, new apps, tweaks to older programs and numerous under-the-bonnet changes.

But that doesn't make it necessarily a good choice for some users, no matter how high their enthusiasm.

In fact, Apple has spelled out hardware requirements for Leopard that will leave some users of older computers in the dust - unless they opt for new hardware. That doesn't even include those who still use the "Classic" Mac OS, once known as Mac OS 9, or apps that haven't been updated yet to run in Leopard.

It also doesn't deal with the fact that some people are simply averse to change, wary of jumping on the introductory release of any operating system. Just ask all those companies patiently waiting for the first service pack for Windows Vista.

With that in mind, here's a quick look at some of the things to consider when deciding whether now is the best time to upgrade your Mac.

1. Older hardware. If you're using an Intel-based machine, grab the credit card and head for the Apple Store. Leopard will run on all Intel-based laptop and desktop Macs, meaning all of those released since January 2006. The same goes for older hardware that uses the Power PC G5 processor; you're good to go.

However, if your computer runs on a Power PC G3, forget it. And if you have any hardware running on a Power PC G4 chip at anything less than 867 MHz, ditto that advice. According to Apple, Leopard isn't for you. You'll also need something with at least 512MB of RAM, a DVD drive and about 9GB of space.

2. Mac Classic. If you're still using applications designed for Mac hardware before Apple made the leap to Mac OS X more than six years ago, you have a choice:

  • You can keep using those apps with Tiger - and eschew Leopard until replacement software comes along.

  • You can ditch the app if there's an OS X version available and move to Leopard.

  • You can install Tiger on a separate partition of your hard drive, use Classic there when you need it and install Leopard on the other partition.

Apple is clear on this: Support for Classic is being dropped. You can't say you weren't warned: Apple CEO Steve Jobs himself proclaimed the death of Mac OS nine several years ago.

3. Software compatibility. Although Apple has a reasonably good record on backward compatibility whenever it pushes out a new operating system, an unknown number of apps are still likely to break. Unconfirmed reports online about pre-release versions of Leopard, for instance, pointed to Cisco's VPN software as one such application.

That means either living without those programs until updates are available, or waiting to upgrade. Users who aren't sure what works and what doesn't should check with their software vendors first. And it's likely big-name vendors will push out updates for their apps fairly quickly.

On a related note, anyone with software that modifies Apple's kernel extensions should hold back on an upgrade; those kernel extension mods will almost certainly have to be rewritten specifically to work with the new OS kernel. It's also unclear that hardware drivers specific to Tiger or previous versions of Mac OS X will, in fact, work properly with Leopard. Drivers have broken with every new version of Mac OS X - or any other operating system, for that matter. Caveat emptor.

4. Fear of change. Everyone always says you shouldn't buy the first-year model of any car, nor should you get .0 versions of software. Wait for the .1 or .2 release, goes the advice. (Although Leopard is Version 10.5 of Mac OS X, it's really the equivalent of a 5.0 release of other software. So in this case, the advice would be to wait for Version 10.5.1 or 10.5.2.)

No doubt, this operating system, like all others, will have bugs not yet found or squashed. If you're the naturally cautious type, why rush headlong into an upgrade, especially in a work environment, when you can wait a few weeks for the inevitable update?

This was an issue for some with Tiger - and is especially important in a business environment, where broken apps can mean lost money. Anyone burned by the move to Tiger in 2005 will no doubt be chary of moving to Leopard too soon.

5. Tiger is "good enough". If Leopard doesn't offer any immediate must-have software or changes, and everyone at the office - or at home - is chugging along just fine with Mac OS X 10.4, then there's no need to rush an upgrade now. This is especially true for users who are planning to buy new hardware soon. Why spend $129 now when Leopard will be included for free with the next Mac you buy?

The bottom-line advice: If it ain't broke, buying Leopard won't fix it. But if Leopard offers useful tools, doesn't break older programs and your hardware is up to date, the choice is yours.