When it comes to something like this there are only a few choices:
1) Get an "old" iMac from the Apple refurb store. You'll get a good deal and the screen you want. If however there is some feature of the new iMac you have to have (FW800, the faster CPU, the 4gb RAM, heck, if you just like the look if it) then you go to option #:
2) Get a Mac Pro/mini + screen of choice. The iMac screen just doesn't cut it for you, so you get a headless Mac and a nice display. But, if the mini isn't powerful enough for you (crappy HDD, GPU, etc) and the Mac Pro is out of your price range (they start at $1900 refurb, no screen), then you go to option #:
3) You get a 24" iMac. Pretty obvious here, you get all the same "guts" as the 20" but a bigger, better LCD. But, if you dont' have the $300 extra or can't fit a 24" iMac in your workspace, you have to move to option #:
4) You get a new iMac (2.4ghz model, as it has 256mb VRAM) and buy a second LCD. You can get a nice, 24-bit Dell 20" widescreen for about $300 if you wait for a sale, buy refurb, eBay, whatever. Same resolution and size, and has adjustable height stand so you can match the level of your iMac screen so it'll be a comfortable setup. You can use the iMac screen for tool bars/dialogs, web/email, multimedia, whatever, and move things where color is important over to the external LCD. But, what if that $300 is just too much for you, or you don't have the deskpace, or just hate dual displays? That leaves you with option #:
5) You're screwed. You're going to end up with a non-optimal setup, so you either have to settle or wait and save up more money to get what you want/need.
I should start a decision consultation business.
Hope that helps.