"...the screen is awesome and much better quality than the one on the new 20" iMacs"
I don't know about that, Paul. I have both, and prefer each for different uses, as we have a business that we use them in. I wouldn't say either is better than the other. We have 3 20's, 2 24's, and (I think) it's a 17 (one of the first round of intel iMacs

) I've never had a screen problem with any of them. The 17 is on every day, 12-15 hours a day, Monday thru Saturday. It is over 2 years old and still looks great!
I do LOVE the size of the 24, but in some cases, it is too big

Seriously, in cases where I have to sit a bit closer in the field, the 20, hell, even my 15" MBP is a better choice.
One reason to go with the bigger screens is also the video card, the chance to game a bit more (or with Windows/BC).
I wouldn't say the Nvidia choice is necessarily the better choice, especially if you are doing Photoshop/Final Cut/etc. There isn't going to be any performance gain in creative apps, and in some cases, it can have a negative effect.
That said, I love the iMac line. Since buying them for our business (replacing expensive laptops) my Mac Pro has been neglected! They're awesome computers, for sure. If you have a 20 right now, you have an Intel iMac, right? Therefore, there should be NO reason to upgrade right away. You may as well wait to see if something changes in January. Even if you decide to stick with the older (2008) version when they release, you will be able to save a bundle in the refurb store when the new ones drop.
Peace
J