What wattage is your Power Supply? To find out, just take the side panel off your case and have a look at the sticker on the power supply. It will have all the information about it. Unfortunately, there's no other way to do it. There's no software or anything like that which will tell you the specs of your power supply like there is for your other components.
If you're gonna get a new video card you're going to have to take the side panel off to fit it anyway right?
If your power supply is fitted in such a way that the sticker is not viewable, you'll have to unscrew the PSU from its housing and look at one of the sides that was hidden. It's only a few screws. You can do it.
If you tell me that information as well as your budget and intended use, I can find you a suitable Graphics Card.