It really depends on your usage. I "stream video" as well, but even with most of it over 3G, I have barely used 1GB in a month. I primarily use Netflix and the ABC Player and have found an hour long show over 3G to be about 100MB. So, that's roughly 20 hours of video or close to an entire season of an hour-long TV series. Usage will of course vary, but I don't watch nearly that much in a month apparently.
Aside from a rare few apps, every one I have downloaded has been relatively tiny - 15MB or less.
Compared to video, most email and web traffic is also pretty minor in my experience at least. I do most of my web surfing over 3G, and it may offset an episode or two at most.
I haven't tied audio streaming yet (decent sized iTunes collection instead). I could see that adding up mainly because it's easy to have it go for hours upon hours. But I don't know how much data it would chew through in an hour.
*shrug*
So, personally, I was worried about my limits, but have found them to not be an issue at all. However, I'm not 100% 3G, but the majority of my heavy usage is 3G. The only real limits I have found are speed issues. Trying to watch videos at free public wifi locations or in a couple areas of low 3G coverage (for me, typically inside large buildings) can lead to an annoying amount of buffering.