That 20" iMac is solid for running at least a dozen apps concurrently. Including Photoshop. The machine is great of course, right up there with the best of 'm to be sure, but the secret is in the operating system: OSX. And to run that you're not limited to Apple computers anymore. You can now install and run OSX on 'ordinary' PCs:
http://dailyapps.net/2007/12/hack-attack-install-leopard-on-your-pc-in-just-one-step/.
You can even install OSX on netbooks. Though these lack the power, graphics, and memory that you want for photography/editing.
On a
real (i)Mac, though, you can run every operating system you want with
VirtualBox (FREE!): OSX, Vista, XP, Ubuntu, etc. etc. And at the same time, side-by-side on-screen, if you like. Try
that on a PC!
So you can use all your 'old' software on your new iMac too. Nothing gets obsolete. You really get 2, or even 3 computers for the price of one!