Remember those myths I mentioned? This is another one. The Power Mac line is easily upgradeable. I'm not too sure about the latest iMacs, but someone is bound to make upgrades, just like they did for the ofiginal CRT iMac design. FWIW, I'm currently typing this post on an original iMac running at 233mhz with 128 RAM. Not very fast, especially by today's standards, but I'm running the latest apps with ease. This computer is NOT slow. Okay, yeah its a little slow, but damn this thing is nearly five years old, and its not slow at all for what I do with it (a few games, internet/email, even occasional graphic work, that kind of stuff). My Windows buddies in college were upgrading their stuff quite often. I simply never needed to. I can still easily upgrade the processor, RAM, and hard drive, even on this 1998 iMac, but I never really saw the point as by the time I upgraded it, I would have easily almost spent the money for a new iMac. I want a Power Mac here before too long, but I run all my apps just fine on this one. If I had a Power Mac, I would certainly consider upgrading because it would be cost-effective to do so. Not so with this old thing, or with any iMac, really. But that's the point of the iMac anyway: web surfing, email, a few games; you know, just standard stuff. The main point I'm trying to make is that I simply don't NEED to upgrade, but I could easily do so, even on this machine. The life-span of a Macintosh is far greater than what I've seen with Windows-based machines.
To clear things up a bit: All Macs can have RAM upgraded and all can be sone at home. (BTW, never buy Apple RAM, it's over priced and only had a one year warranty. Buy from Ramseekers.com and go with a company that has a lifetime warranty. ) If you are looking for upgradability, go with a G4 tower. It has 5 PCI card slots not includign the video card. The built in Video card screams already so an upgrade to that won't be necessary for a long time. Hard drives are upgradeable and there is a built in IDE cable for an extra one already built in. If you go SCSI you can get up to four, IDE Apple supports 3 HD's. Unless you are hardcore, you won't be disappointed with options that you can upgrade to. The exception is the processor. While there are processor upgrade cards out there, Apple doesn't support them at all. Also there is no path for upgrading the processor itself with a higher processor speed in the future. Then again with processor speeds around 1.42 (some models with dual processors) you probably won't have to worry about speed for long time. Hope this helps!
And while I know there are a few people on this board who do build their own systems and would have no problem upgrading their motherboards and processors on their windows machines, I know the vast majority of even the windows world does not approach it that way. They will, too, often just buy a new system when the old one doesn't get it done anymore. Of course, I can generally move up to a new machine much cheaper in the windows world than I can in the Mac world.
