If you can afford it go with the 2.4 20" stock. that is what i got and you get a better CPU better Video and a better HD.
so for the extra 300 Dollars you get the following:
.4 Ghz more
128MB more of vram plus a better GPU
70 GB more HD space.
It really depends on how often you upgrade your machine... if you're going to keep it for 5 years, you'll probably want the extra CPU power. On the other hand, if you plan on a new machine in three years, it might not be as important to you.
The GPU/VRAM won't make a lick of difference to you if you're not a gamer. There MAY be some difference in Motion (part of FCS), but if performance in FCS is important for you I would advise against the 20" iMac in favor of something a little speedier anyways.
The HDD space, as mentioned, can be upgraded cheaply and easily later. You can add a 500gb FW drive for <$150 now-a-days.
If you're working with a fixed budget, and want the best performance for your money right now, and you're NOT a gamer, get the 2ghz model, add another gig of RAM (should be about $50), get a 500gb FW drive and take the $100 you saved ($1200+50+150=$1300 vs. $1400) and buy Parallels, iWork, PS Elements, Toast, or any of the other great pieces of software that make your Mac that much more useful. Or put it in the bank and save it for a rainy day, etc.
