A common question I've been asked is, "What version of OS X should I
run on my (insert model) Mac?"
In the past, I always advised users to go with the latest and
greatest. With the diverse mix of machines out there - G3s, G4s, G5s,
and the various Intel-based Macs - it isn't as cut and dry as it once
was. For me, the ideal version of the OS for a certain machine is not
just about performance, but compatibility and use with newer software.
Another variable is what you're planning to do with the computer: If
you're running applications in Classic Mode, Mac OS X 10.5 is
not for you, and 10.4 is as good as it's going to get. (Yeah,
there's SheepShaver,
but that's only worth the trouble if you're using an Intel-based
Mac.)
I'm going to break this down into the classes of machines as I
consider them and the OS that works best on them. We're going to start
from the beginning of OS X capable machines - the beige G3s, WallStreet PowerBook G3s, and
the tray-loading iMacs
working up to the latest and greatest Intel-based Macs.
For the sake of this article, I'm going to assume stock
configurations (aside from RAM and hard drive).
The 10.2 Class
Back in late 1997, when the beige G3s came out, few of us expected
to still be using them on a daily basis in 1998. Many businesses,
academic institutions, and scientists are still using them to this day.
Since the Beige G3, WallStreet PowerBook G3, and tray-loading iMac all
work on basically the same tech, I recommend going no higher than Mac
OS X 10.2.8 on these machines.
Apple limits the Beige G3 and WallStreet PowerBook G3 to 10.2 - and
that's okay. Without upgrades 10.3 would be pretty sluggish on these
Macs. Tray-load iMacs (often 128-256 MB of RAM are present on these) do
best with 10.2 as well. I've had nothing but laggy, moderately
unpleasant experiences with anything higher than 10.2.8 on a tray-load
iMac.
The 10.3 Class
This is, the biggest class of machines by far. I recommend 10.3 for
any sub-800 MHz G4 class machine with less than 768 MB of RAM. The
Lombard and Pismo PowerBook G3s work great
in 10.3, as do all G3 variations of the iBook. The majority of titanium PowerBook G4s also fall
into this grouping. Slot-load
iMacs hit their stride here, as do the Blue & White G3 and the
early Power Mac G4s. I'd say that 256 MB of RAM is the least you need
for good performance, with 512 MB being great.
The 10.4 Class
This class is also big, and it has overlap with the 10.3 class. As a
general rule, I don't recommend 10.4 on anything with less than an 800
MHz processor and 768 MB of RAM. Slower machines do well with extra
RAM. Some examples - a Blue and White G3 350 MHz is horrid with 256 MB
in 10.4, so-so with 512 MB, good with 768 MB, and pretty spiffy with
1 GB.
Any Power Mac G3 or G4 with 1 GB of RAM or more will be rocking
within 10.4. If you're using a Power Mac G5 or iMac G5, 10.4 will make
it shine - it's especially ideal if you need to use Classic Mode for
old applications.
iMac G4s work well in 10.4,
provided that they have their RAM upped to 768 MB - with 1 GB
being the preferable point. (With the iMac G4 - they seem dog slow with
512 MB in 10.4, but the extra 256 MB makes all the difference.) The 800
MHz or higher titanium PowerBooks, and all aluminum PowerBook G4s run
best in 10.4.
The 10.5 Class
I'm going to speak out about 10.5: It works well for some machines,
but not for others. I would say that a 1 GHz single G4 with
1 GB of RAM is the absolute lowest you can go while getting
reasonable performance. From my experience, 10.5 works best on G5 and
Intel-based machines. I only recommend 10.5 for G5 users who don't need
Classic Mode. The biggest beneficiaries of 10.5 have been the folks
with the newer Intel-based machines - performance is, for the most
part, better, and it's a good fit.
These are the recommendations that I've put together based on my
experiences in the field. These recommendations are based on stability,
reliability, compatibility, and overall performance. Your mileage may
vary, but these are based on memory and hard drive being the only
variables upgraded. If you feel differently, I'd like to hear back with
your views. 
