the 580s can use up to ~850W while under full stress, so if your gaming your PC could theoretically be pulling this much electricity from the plug:
however, it almost certainly wont be pulling 850W much of the time. on average its more likely to be ~650W while gaming.
it should be far far lower than that while just browsing the internet and stuff (wouldnt be surprised if its ~150W or less)
if you really want me to i'll see if i can post up something that will tell you exactly how much it will cost to run your computer
*edit*
woah, that cture really fails on these forums. let me see if i can fix that
sorry, just realised that you are probably charged in units, not kWh. also, i doubt you want to look at a fraction as wide as the forums
no problems, if it's much of a jump try seeing if you can switch to a better tariff to save money.
I was on a tariff with EON that was good if you used less electricity, but if you started using more than a certain amount it was cheaper to switch to a different tariff which would cost less for the same amount of electricity (and gas) compared to my previous one.
I suppose the tariffs are confusing though so I don't blame you if you don't want to try
