Good article reminding readers of the important possible problem areas whenever updating OSs or Hardware: cables, ISP connections(especially pesky DSL), hardware drivers, hardware compatibility, and availability/cost of support. Here are some more, hopefully helpful hints, on how to make OS transitionining easier...

TWO OS PARTITIONS, preferably on separate bootable drives (in case one goes down), especially if your business depends on your computer. In one partition have the current OS that you know is working, in the second partition, build the new OS, collecting drivers for your hardware as they become available, and getting it working. If one of those partitions is a flavor of OS9, keep that separate from the OSX/Classic partition, i.e. don't try to use your old working OS 9 partition as your Classic partition.

With this two OS partition strategy you always have a place to fall back on that you know will work in a pinch. And it allows you to spread out the pain :-) of transitioning over a longer period. If your new machine can't support your older working OS partition, keep and use your old machine up and running as long as you can/need to (network them together for easier file access and sharing).

HARDWARE COMPATIBILITY AND OSX 10 In general if your hardware doesn't have either a USB or Firwire connection, then it will either be impossible to use, or you will likely have extra problems in getting and keeping it working. Even if it does have a USB connector, driver support may not yet be current with the latest OS update, check the companies website support areas (and other support websites e.g. MacInTouch, MacFixIt, etc.) for latest info on driver problems / updates.

For OSX besides CUPS for older printer support; there is also LaserSoft SilverFast SE ($49) available for many makes of scanners (notably missing are Canon scanners). LaserSoft has in general been quick to update their products for new OSs, and in many cases their software will be just as or more full featured than the scanner software that came with your scanner. For older digital cameras (that use ADB/Serial connections) but have media cards, consider an inexpensive card reader that works with OSX (check the website to make sure that drivers are current).

BACKUP/ARCHIVE your and project files, at the least, and your system partitions too. I've found using an external Firewire drive that is as larger or larger than your internal drive(s) to be a fast, easy, way to do daily backups of my work. An external Firewire drive is also easily transportable (especially if its a "pocket" model drive) for moving your project files to or between other systems when the need arises.

SUPPORT Unless you have toll free access, a speaker phone (allows you to put the phone down while waiting for the support person to answer or get back to you) and lots of time on your hands, make the many free online support boards and forums your first line of support and trouble shooting. Often times a quick search of those areas will reveal that someone else has already spent the 4 or more hours finding the solution to your problem and has kindly posted it there.

i have a G4 450Mb 20G harddrive and yes, it's 2 years old. recently I installed an internal dvd burner (Pioneer) that was not supposed to be installed in my particular mac.but it runs beautifully with dvd studio pro and producing dvd and inserts as well. Okay it runs a little bit slower but I saved myself a couple of hundred $$.
I also use photoshop 7, golive 6, illustr.10 with NO PROBLEM at all with my jaguar.
Sounds you have a lemon