You'll be fine with 500MB I'd say. I think, unless you go way over on a regular basis, that most networks will just turn a blind eye, but don't quote me on that.
I'm not sure how much data youtube actually uses but, like I mentioned with images, you don't get a top quality, high definition version of the video when using 3G. Your usage sounds pretty much identical to what I use mine for when I'm out, ie. I'll be sitting having a discussion with friends about something and google it to find a conclusive answer
I've just checked mine and I admit I'm not a heavy internet user on my phone but this is how my 3G internet usage works usually.
- Possibly downloaded an album or two over 3G,
- Use Twitter quite regularly
- My email is set to PUSH so it alerts me of emails almost as soon as they're in my inbox
- I often use the Maps as a quick and easy route planner/checker when out and about
- Quite often leave eBuddy connected 24/7
- I use RunKeeper Pro to live track my runs (so it follows my route via GPS and sends the info to the RunKeeper website over 3G/GPRS constantly, which is usually around 45mins a time).
- Checking Sky Sports News and Score Centre via the respective apps.
- General browsing like I mentioned before
And since November I've only used 465MB of data (65MB sent & 400MB received).