New iMac or Tower???

So much else changes in a period of 3 years, that even if you COULD change the CPU, you would still have a machine filled with old technology. One of the benefits of the tower, of course, is the PCI slots. If FireWire2 (or whatever they finally call it) is released in a day, or a week, or a month, then you have an iMac that will never have it. With a tower, you wait a month until Orange Micro or Belkin, or someone releases a PCI card, and you add it to your tower. More fundamental changes, though, like bus speed, memory bandwidth, etc, are not solvable by adding cards or swapping CPU's... in the Apple world, it pretty much means a new computer. I do not see anything truly revolutionary being released soon enough that the average consumer would be overly vexed by having an iMac. Yes, in another 3 years, you will want a new one. That's still twice the lifespan of most PC's. (At least those used for gaming, etc.)

Everycomputer is slow feeling after 3 years

Well, personally, I would wait for new towers (if time permits) because when they come out, they will be much faster than the new iMac (they need to be to be considered Pro machines).

But if you need to upgrade now, then get the cheaper iMac. When it gets too slow for you (which it will, as all computers do), upgrade the computer again (~3 years). If you need the expanablity, then you have no choice but to get the tower.

Also, you will probably like a tower with the massive cinema display a lot more than the iMac. But that's my opinion.

Tomasz