Normal surfing you should be okay. Youtubing a lot, no. Downloading a lot, no. With Xbox Live, I'm not totally sure. I'd say you would be pushing it with a lot of Xbox Live, though.
At home here we have 100GB/month transfer and some months we've had trouble keeping under that. Keep in mind that's a lot of streaming and some large downloads, too.
Why did you buy a Rocket Hub instead of just getting a normal internet package with faster speeds and higher usage caps?
I got the hub because they recommended it and I needed wireless internet in my house. And there was not a spot to connect the cable where I wanted the computer. There is no going back now anyways -.- (for 2 more years) But they will offer more gb in the future I asked them so it won't be so bad. :)