I also had same query but i found solution
You can use BitMeter. Its freeware
& alert you when ur download limit will near to exceed. I also have virgin connection but with only 2 GB limit
You can also use Bandwidth Meter & netmeter, but its trail version. It have more advance features like email, sound alert etc.
I am using both bandwidth meter on trail & bitmeter. If bitmeter will fulfill my requirement then i will through out the bandwidth meter, why should i waste my money on this?