I want a tower (mid or full) because I don't want to be tethered to a monitor. Yes of course you can add a 2nd monitor to an iMac but what do you do then if you want to use the 2nd as the primary and not use the built in at all? Put the 2nd in front of the iMac? That's an awful lot of desk space going to waste.
I like iMacs, they're great computers. I was going for a 24" but I think I've settled on the 2.66 Mac Pro. I'm wishy-washy as to whether or not I need all that power, but I do know that I'd love to have it (+ 2 SuperDrives) for ripping/encoding and burning dvds, so...I'll pay the extra to get it.
I do wish there were more choices, but I don't think we'll see it anytime soon. Seems like Apple believes that most users can either use a Mini or iMac and those who need lots of power will get the most out of the Mac Pro. The problem is, they're forgetting the people who want the expandability, not necessarily the power of a Mac Pro.
As for the question on upgrading a current system -
granted, the extent of my upgrading has been HD, but nevertheless...I went from -2- 80gig ATA Samsungs to those + a 320gig SATA Seagate in my XP PC. I refuse to put money elsewhere into the computer because I will be buying a Mac Pro in the next year. I don't PC game much at all so there's no reason to upgrade from my Radeon 9500, besides my mobo does not have PCI-e and AGP video cards are dinosaurs at this point in time. My 500w PSU is adequate (always has been), my 8x LiteOn dvd burner is adequate enough...1gig of ram is adequate enough (I'd like to have at least 2gigs but again, I'm not sinking money into this computer any more unless it's something that can go into a Mac Pro like the 320gig SATA Seagate I bought recently).