I was originally skeptical about the difference between the displays in the 20" and 24" iMacs, but they turned out to be significant. I couldn't believe Apple would even use an inferior monitor in one of their products - but they do. It's a factor not often mentioned in spec comparisons where the emphasis is on operating performance. If you don't use the computer for color critical tasks, like photo editing, video processing or games, then the 20" screen is probably sufficient. Absent these kinds of work, the added RAM and hard drive capacity and processor speeds are probably irrelevant as well. But if you do any high-end work on your computer, the 24" screen is far superior, both as to color quality and working area.
As for moving application palettes to a second monitor, I used to do that in the old, CRT, days. But once I tried my first Apple wide screen, a 22" Cinema display, I was hooked - and wouldn't go back now if I got a second monitor for free. I now use a 24" NEC monitor with my Mac Pro. The difference is in the ergonomics - one wide screen requires less neck straining head turning than two standard monitors. Certainly there are situations where multiple monitors are necessary, for instance for stock traders and others who need access to multiple spreadsheets at once. But for Photoshop and Final Cut, one wide screen monitor is an excellent solution - in my opinion. Not to mention that it uses up far less space on a desk than any two monitors.
In respect to the lack of a significant CPU speed boost in the new iMacs, I suspect it is due to the pending availability of low power quad core Intel chips. They should show up in MacBook Pros later this year and will probably make their way into iMacs as well. Within a year, they will likely be the de-facto CPU in mid to upper range laptops and desktop computers like the iMac. One flavor or another of the core 2 duo has been around for two and a half years now, which is a long lifespan for any CPU chipset. Which also suggests that, if you don't absolutely
need a new Mac right now, it's a good idea to wait for the CPU refresh. Likewise, OS X is due for an upgrade soon to Snow Leopard. Waiting a little longer to buy a new Mac means not having to pay $129 for the new OS.
The glare problem with the iMac is another issue that gets too little attention. One poster suggests that you get non-glare film for a glossy screen, but these solutions are not cheap (see:
Photodon), nor easy to apply. On the other hand, my 24" NEC has a standard non-glare surface (not glass). Apple's move to glass in their laptops is a reasonable choice, given the added rigidity and strength glass provides. But, since they now offer a non-glass, non-glare screen for the 17" MacBook Pro, one for the iMac doesn't seem beyond reason. Admittedly, it's not as handsome as the glass screen MBP, but it is every bit as functional, if not more so. IMacs are commonly used in educational institutions where overhead lighting is commonly far less than optimal for computer use. A glossy screen iMac in a room full of overhead fluorescent lights seems to me to be an ergonomic nightmare scenario. Since Apple sells heavily to schools, I just don't understand their unwillingness to provide non-glare iMac screens - or access to non-glare film solutions, at least. Fortunately for most schools, these days they are unlikely to be able to afford an upgrade to glossy Aluminum iMacs. Maybe by the time the economy recovers, Apple will have come to its senses.