Well you won't get anywhere near 5GB (Gigabytes) just playing online games - after all it's the equivalent of downloading over 500 mp3's a month.
The amount of data you can download with a 3G or 4G mobile connection is not what you should be concerned about, what is more important is latency. While mobile Internet may be fine for browsing the net, it can really be troublesome for online gaming, where ping times can make the massive difference to your online experience. Latency on mobile Internet can be very high, making online gaming in many cases almost impossible. A good cable or DSL connection ping is about 50 > 75MS, however 3G and 4G connections go into the hundreds, which is not good for games like Black Ops.
Also, you can't connect your Xbox direct to a mobile Internet connection, you need to 'bridge' it first with a PC.