Great great website that helps me find my estimated wattage is http://www.pcpartpicker.com
This website you can put all your gear in there and it will give you an average rating for your wattage. I do tend to go a good 100 watts of what it says just to be safe about it. Also your computer will work to upgrade to a 760 but there's a catch you might experience some slight lag or not feel your getting the max out of your graphics card. The reason being is that the PCI slot that the cards connect to are 2.0 in your computer and the ones on modern day cards are 3.0 so as I love to make and build computers my suggestion is get a new board but if that's out of the range then just live with a slightly not at a 100% card. But to be honest you yourself might not notice because your upgrading for generations ahead so... Well that's all I have. Pm me or any of that matter or find me on teamspeak I am very knowledgeable when it comes to computers and can answer almost anything.