Is this mobile broadband as in using 3G/your mobile? If so, you'll probably only be playing about 5 minutes of CSS/TF2 before you quit with disgust at your 500+ ping.
Otherwise, it depends on things like your rate settings and is generally quite easy to work out from those. Given the fact that even 256kbit upload ADSL/Cable is more than ample (and I seem to remember 128k being okay a few years back too) for gaming, it won't exactly burn through your bandwidth allowance, though it'll certainly make a fair dent in it if you play a lot.