I guess for upgrading, I was thinking more of upgrading the actual processor. That's usually what gets me to buy a new computer, and replacing a motherboard is usually more of a hassle than buying a new machine. I generally tend to try to futureproof my machines when I buy them in terms of peripherals. I kind of want to avoid buying any computer until (if) Blu-Ray becomes standard, and maybe OLED screens are available.
On the one hand, you can resell an older iMac if you can't repurpose it (their value holds up really well: I sold a 3 1/2 year old iBook for $500 once), but it is kind of lame to have to buy a new monitor all the time. The Mac mini can only power 1 monitor at a time which rules it out for me, and the Mac Pro, even the lower end one, is really aimed at people who are willing to spend. (It is, by the way, far easier to upgrade a Mac Pro than any PC I've ever seen. Drives literally snap into place, there's plenty of room, etc.)
So for that reason, while the Mac is a definite home run for laptops, the lack of a mid-range tower or pizza box (more upgradable than the mini but less high-end than the pro) does make the calculation there a little tougher. Still worth it, I think.
