Lots of ways to do this, some simpler and more efficient than others.
The most transparent way is to invest in a paid Internet-based synchronization service like SugarSync (
http://www.sugarsync.com). They sell packages with storage capacities up to 250GB, which would be helpful, given the type of files you're talking about managing. Because of the sizes of your files, though, this may also be slower than you would like.
Apple's .Mac service ($99/yr) comes with an Internet-based storage solution called "iDisk". Similar in functionality to SugarSync, except that with iDisk you have to store the files you want synchronized in a particular location (the iDisk volume), whereas with SugarSync you can tell the application which directories, anywhere on your hard drive, you want to keep in sync. iDisk is limited by default to 10GB of storage, with paid capacity upgrades available.
Time Capsule is an excellent
backup solution, but it isn't so handy for
synchronization, because Time Machine doesn't always let you "restore" files to a machine from which they weren't backed up. Not sure under which circumstances this occurs, but I've encountered it myself when trying to "restore" files from a MBP backup to my MacBook Air. (I'm not talking about while installing the operating system, but rather during regular use.)
An external hard drive is a good solution, but you have to be sure to avoid synchronization conflicts of the type where you edit a file on one machine, forget to synchronize to the other, edit the file on the other machine and then sync. You've just lost your first set of edits.
ChronoSync (
http://www.econtechnologies.com/site/Pages/ChronoSync/chrono_overview.html) is a nice program for keeping files in sync, and you could setup a WebDAV server (such as CrushFTP,
http://www.crushftp.com/index.html) on the iMac and then tell ChronoSync to synchronize each time the DAV server was mounted. This is more transparent than the external hard drive solution, but it carries the same risk of overwriting previous edits. (It does have an option to archive overwritten files, though. If you were careful enough with your setup, this problem could be avoided.)
I've tried all of these. For my money and time, SugarSync fits the bill. Their Mac client is still in beta but seems quite stable, and their customer service is not bad at all. I recently reported a minor bug in their software on a Sunday morning and got a reply directly from their CEO within a couple of hours. Their platform leader remained in contact with me for about a week, at which point they issued an updated release that fixes that particular bug.