I see myself in a lot of these posts - someone who came from the PC world and was USED to a desktop system that's fully expandable - RAM, hard drives, DVD drives...
When I was looking at my options, I said, "Ok Mac Mini, cool, but way underpowered and incredibly difficult (but at least *possible*) to upgrade."
Next the iMac - "Well yeah, technically it'll do most if not all of what I need, but I'm not comfortable with the all-in-one design...what if the screen dies, then I'm without a computer for a couple of days minimum - not cool."
Technically as a consumer and not expecting to use the Mac for "pro" reasons, those are my "only" options.
As a power user though (and even on the PC I did this), I keep on average 6-8 programs open at any one time - at least one of those apps being Firefox with...well, at the moment I have 11 tabs open, but usually no less than 7 or 8.
One thing I can say about the Mac in general is that at *least* the programs tend to be smaller and more efficient than Windows programs, but I still average using 4-6 gigs of RAM, which is why I have 10, and am considering getting an extra 4 just for the safety factor.
If Apple would offer a $1500 xMac, that's upgradeable like the Mac Pro, I most likely would've bought that instead. And I can't see how it could possibly cannibalize Mac Pro sales - Pros know they need those high-end XEON processors for processor intensive work. Set up the xMac with an i7 chipset with an option for 4 ($1500) or 8 ($1800-$2000) cores, and it would seem to clearly delineate the two computers.
If anything, the xMac *could* eat into the sales of the higher end iMacs, but even there, you have the people who want the *OPTION* (I don't think Apple knows what this word means) to choose what screen they want. Not everyone wants a glossy, color-rich computing experience. Some of us (like photographers, retouchers, etc) want matte screens so we can accurately see how an image looks so we can properly color-correct / retouch / edit it.
As Blue Velvet also said, you should always buy the best you can afford at the time - the better and more powerful system you can afford, the longer you can keep it around and make incremental upgrades.
I've had my system for just over a year now, and with the state of technology now, I can see myself owning this machine for at *least* another 3-4 years. Certainly Snow Leopard should help to take advantage of more cores during that time, and when Adobe finally rewrites Photoshop CS5 in Cocoa (and full 64 bit), it will just be that much better.
And when I sell it, I should be able to make back enough that I can at that time afford whatever the newest Mac Pro is, and go on from there
To me, or hopefully to most of us stuck in between the iMac <-> Mac Pro decision, the Mac Pro is the better *LONG TERM* value
