Nearly all of them are going leave their machines alone. Apple is right that RAM is the most common upgrade for your average user. Even then it's terrifying for an iMac owner. (Perception not actual difficulty.)Note: Please keep in mind that we're trying to consider what may very well happen to a machine over its 5-7 year lifecycle. The iMac isn't particularly expandable, so its a lot easier...but how many PC home users can really resist the temptation of leaving it alone? Given all of the upgrade parts that one finds in aisle after aisle after asile in Best Buy, its hard to claim that there's utterly no market for PC upgrades.
You'll hear me go on about how the iMac doesn't have any expansion. My solution is not to buy an iMac. Nearly everyone else in this thread has had experience with a wide variety of machines from a GAMER rig to workstations. The iMac isn't what we're looking for and we accept that.
We also accept that the iMac is going to be on the lower end of power consumption as well. We just don't understand how the iMac consuming less power by orders of magnitude. If you're running it at full load 24/7 you are wasting your money on its paltry Core 2 Duo with 2007 speeds.
Get a Core 2 Quad, Phenom II X4, or Core i7. You'll get more work done per watt and get it done faster. In my experience scaling from a 2.4 GHz Core 2 Duo to a Quad you are getting near double the performance.
Whenever I get a support call it's a 2.4-2.8 GHz Pentium 4 Northwood with 512 MB of RAM and Windows XP. That's my generic standard for what everyone is using today. These machines are hitting 5 or even 6 years too and have never been touched.