How much is that 2nd monitor gonna cost you? The 20″ iMac is only $400 more and you wouln’t need a 2nd monitor. Probably easier to carry around than a dual-monitor setup. I’ve got a 20″ and the screen is fantastic. For the kind of work you’re describing I can’t imagine you’ll have any trouble using that and that alone.
Not to mention that the $400 will get you a faster processor as part of that price.
But yes, the iMac will make you happy. I’ve been using mine to do multi-cam DV editing and it’s done very well. IF you get the 20″ I suggest upgrading the video card ($75), the RAM to a 1 GB chip ($100), and another 1 GB chips from crucial.com ($189). You can do without the video upgrade if you want to save money but you should get at least 1 GB of RAM at a minimum.
To expand on what Jerry said, you’re going to need the Universal version of Final Cut. You’ll either have to buy a fresh copy (so, a 3rd for you) or pay $50 to crossgrade one of your current ones. That involves sending Apple the discs and you’d have to take it off of one of your other computers. So long as you do that part it’s perfectly ok, however. Search Apple’s website for “Final Cut Crossgrade”.
Also, if you’re gonna be traveling, you should get a case for the computer:
https://eshop.macsales.com/item/LTA%20Projects/851236001091/