I would not recommend playing COD over your 3g with a 10 Gig limit. To give you a little perspective on how much data gets passed around when you surf the net...
I work from home every now and then, and I also use a transparent proxy. The proxy also has a few plug ins that I will not go into, but in 8 hours I was able to accumulate 9 gigs of data from port 80 alone (surfing the net).
Now a game holds a constant connection, and it has to transfer large amounts of data to keep everything in sink... You are going to hit that limit really quick! My guess would be an hour or two you will be very close to using half of your limit.
**Please note that this is just a wild guess, I have not measured that traffic throughput from a game.