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Computerworld - Put down the credit card. Turn away from the Apple Store. Of course you want to jump on the latest Apple Inc. 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-hood changes.

But that doesn't make it necessarily a good choice for some users, no matter 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 9 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 prerelease versions of Leopard, for instance, pointed to Cisco's VPN software as one such application.