of an Apple Macintosh is longer than other PCs. Typically, a "PC" has a useful life of about 3-4 years. Macintoshes can last 4-6 years or more (at least, that is my experience in the corporate world and at home, but it depends on what your requirements are).
All of the machines you list, if they were built in the last couple of years (so either high-end G4, G5 or Intel-based) can edit High Definition video.
The "key component to video editing" will depend on your requirements, but it should be understood that there is no one "key" - there are many pieces that make up the entire system - and all have their job to do.
For example, if getting the information INTO or OUT of the machine is your priority, then CPU is key (presuming FireWire is used for the actual transfer). If getting as much of the editing job into the machine is a priority, then a large hard drive is key. The video processing in the machines you list are up to the task of editing HD video, so that is a non-issue. Since you have indicated you are no longer on the high-definition video track, all of the current machines you listed will handle standard definition video editing without breathing very hard.
If you were to get a higher-end G4 to edit standard def video, you will be fine... but what you will find is burning the DVD (rendering the video) will take much more time than if you use a current machine with a much faster processor.
I still have the G4 tower (867 MHz, I think - now about 5 years old) that I used to use to edit Standard Def video from an Canon Elura 60 (and an old analog RCA full format VHS camcorder via analog-digital bridge). It is a great machine. I can still edit standard def video on it if I want. It will import standard def video in real time. But when it comes time to burn the single layer DVD (about 105 minutes worth of video + DVD menuing with motion), I used to start that rendering burning job just before going to sleep at night. Most times, the DVD would be ready when I woke up in the morning. I have every confidence that this G4 tower could handle hidef video too - if I am patient enough - but there is no need for me to do that.
I now use a 2 GHz flat panel G5 iMac. It behaves great when importing standard def video (real time, just like the G4 tower). It is a little over a year old. For High Definition video, when importing, it caches the video from the camera and processes it at about 1/2 normal speed - so it does not import High Definition video in real time. If I have a full miniDV tape (1 hour) to import, I just start the import and walk away - a couple of hours later, it is done importing and the computer ready for me to start editing. Once in the machine, HD video editing is real time and works great. Now, on DVD burning - it takes a couple of hours to render, but certainly not over-night (and more video = more render time). Plus, the newer machines have double-layer DVD burners - whereas the older machines as single-layer only.
There is some time delay when applying and rendering transitions, special effects, titles and credits - and obviously, the machine with the faser CPU will render thes activities faster. But typically, 1/2 second cross-fades or a 5 second title or 20 seconds of credits take only seconds or a minute or so depending on what needs to render. Increasing what needs to render (i.e., 15 second cross-fade or 20 second title or 5 minutes of credits will increase the rendering time to add that effect to the video - but does not impact export or burn to DVD time - that I can tell.)
My son recently got a flat panel Intel iMac. He's at college and I will be visiting him this weekend. One thing I've told him I'd like to try is high def video importing - just to see how much difference there is. I'll let you know what happens.
In the meantime, if you don't have enough hard drive space to hold all the video you want to edit, it does not matter how much RAM you have, what speed the CPU is or what sort of graphics processing is in the machine... and suddenly, the hard drive storage space becomes "the key component"... (so an external FireWire drive of at least 250 gig - bigger is better, especially if HD video is edited - is highly recommended).
The faster CPUs will render the video faster when importing (which we know is not an issue with standard def video) and exporting (or burning to DVD - which we know is an issue). For importing standard def video, all will be about the same - real time - but rendering out the finished product will happen faster with the faster machine.
At that point, the "key component" is the person at the machine, and not the machine itself... but this is just my opinion.