I used to watch an entire episode of Scrubs on Megavideo with the old "mobilecinema" jailbreak add-on almost every single day exclusively on 3G, and never got past 3GB. I doubt you'll be on 3G enough to beat 5GB. The rumors I've heard is that there's a "soft cap" at 5GB, where your speed is throttled, rather than a hard cap, where you would have no connectivity at all. I don't even know if the former is true, but I've never heard of anyone getting service cut off, or any overage charges on the unlimited plan, no matter what their usage was.
Incidentally, how is the quality over 3G? Any hiccups at all? Any blockiness to the video, or noticeable resolution downgrade?