You shouldn't think of

TV version 1 as a storage device (even though you can sync movies to it's internal hard drive and/or upgrade that hard drive to create even more space). Syncing means that a
copy of the movie is pushed to that

TV hard drive; it does not mean you can store 1 single copy there and then delete it from your iMac. This approach will work until the next time you want to sync (say- to push over a new movie) at which point the deletion of the movies from your iMac will sync the deletion of movies on the

TV (you'll have no copies of the movies left).
What you need- either way- is a big media hard drive to attach to that iMac, move all those movies out to that drive to free up the space, and then go with either

TV version 1 or version 2 for how you get the movies from
that (new) drive to your TV screen.
There are a few other options (modestly more complicated) such as jailbreaking version 1 so that you can attach a USB hard drive to it to give you more space than internal drive (upgrade) sizes. The general concept of both version 1 and version 2 is to keep your master (media) files
on your computer, not delete them and have only 1 copy on version 1.
If you have 300+ movies at about 2GB (I'm guessing) each, you need at least about 600+GB of external storage for them. If you want to buy some space for future movie additions, you probably should look to about 1TB or more. just did a quick search to see that a variety of 1TB external firewire drives are under $100 right now (and 2TB drives are under $130). At about 2GB per movie, a 1TB drive would give you capacity to grow your collection up to about 500 movies and 2TB would give you space for about 1000 movies.
My suggestion, buy 2 such drives so that you can backup your movies (or maybe a 1TB for movie collection storage and a 2TB+ for Time Machine to automate the backups). Then, either an

TV version 1 or version 2 can be your transport of movie files to the television AND you'll free up all that media space from your iMac (and maybe add a bunch of additional free space to boot on your new external drive).