I'll spare you the science lesson, and skip straight to maths. A lightbulb uses 40-100W of power. In the UK, the power grid is split down like this:

• nuclear: 19.26% @ 16 g/kWh CO2
• coal: 33.08% @ 891 g/kWh CO2
• gas: 39.93% @ 356 g/kWh CO2

This tells us that overall, we produce 16*0.1926 + 891*0.3308 + 356*0.3993 = 439.9752 g/kWh CO2. Figures from wikipedia and world-nuclear.

Ok, so if I run my 60W lightbulb (we're talking standard, not 'energy saving' lightbulbs here) for one hour, I've used up 0.06kWh of electricity and it has cost the country 439.9752*0.06 = 26.398512 grams of CO2. Ok, sounds a lot for a single lightbulb to be producing in gas, but let's compare this to the amount a single human breathes out in that same hour. Now, wikianswers says 2.4 grams per hour for a human being, and micpohling says 462.2916667 grams per hour for a cow (average of the four figures from the link divided by 24 hours in a day multiplied by 1000 grams in a kilogram). So a cow produces the same amount of CO2 as 17 and a half 60W lightbulbs.

There's a statistic for you! Eating beef is good for the environment. Every cow you kill is the same as turning off 17 and a half 60W lightbulbs -forever-. Stick that in your vegetarian environmentalism pipe and smoke it!