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But the older PC's cannot handle the newer OSes for Windows.

Any Pentium 3 class machine, with a dirt cheap RAM upgrade, can comfortably run Windows XP. Any P2 class machine, with a similar RAM upgrade will run XP (sans some eyecandy) usably, albeit not fast.

That's going back seven years. A seven year old Mac probably won't even _boot_ a current version of OS X, and from having spent about 3 months in front of a 233Mhz Beige G3 running OS X at a prior job ~18 months ago, I can assure you it won't even run *close* to as responsively as a similarly aged PC. Hell, my ca. seven *month* old iBook I find to be only usable for light tasks - as soon as you load it up with any large documents or significant multitasking, the whole show grinds to a[n even slower] crawl.

Alternatively, if you want to extend the life of those old PCs even more, invest in a Terminal Server and use them as dumb terminals. You can install Windows 2000 or even XP on Pentium *1* class machines with 64MB of RAM (ca. 1995-96). Sure, local interactive use will be appallingly slow, but once the TS client has started that's irrelevant. This is not even an *option* with old Macs because OS X doesn't (yet, I'm sure it will eventually) have Terminal Server-like functionality.

Now on the other hand, I have heard that the older MAC's can run the newer OS X pretty well.

You probably have heard that, from Mac users who are acclimatised to OS X's overall slowness. *If* you are accustomed to that unresponsiveness, then an old Mac is probably usable. However, if you're used to Windows or, in particular, lightweight X11 window managers, then OS X on anything short of a G5 class machine is going to feel like it's always a step behind your every move.

Go and load up a few nice big (500 - 1000 posts) Slashdot threads into Safari and watch it crawl to a stop with the spinning beachball of death on *current* Macs like iBooks and eMacs. Observe how right-clicking an icon in the Dock gives a noticable delay before displaying the menu. Try and resize some windows. Now think about doing that on machines anywhere from 1/2 to 1/5 the speed, often lacking the advantage of Quartz-Extreme accelerated video.

Perhaps in terms relative to current and old Mac hardware, OS X on a 5 year old Mac compared to OS X on a current Mac is "less slow" than Windows on a current PC compared to Windows on a 5 year old PC, but that's just because Macs over that same timeframe have increased so little in performance.

"But Expose is so fast !" the Mac zealots cry... And, true enough, it is, but when every other aspect of the interface is slow and chunky, that's a bit of a hollow victory. Like Tog said, the focus of the MacOS interface has switched from usability to "demoability" - making the onlookers "ooh" and "aahh" at dynamically scaling icons and Expose.

Macs don't see more usage because they're any better at running older versions of the OS - they're not. They see more usage because they cost so much more in the first place, companies are loathe to replace them without getting similar ratios of usage/$.