Wattage is listed on the nvidia and AMD cards list on wikipedia or on their sites.
Video cards use the +12V rail so Watts / V = amps. Ie: 120/12 = 10 amps.
If you are concerned about if a GPU will work in your system find the CPU wattage, add that to the GPU you want and about 50W for other components. That will be the amount of wattage you will need over 12V. If your PSU doesn't list wattage over 12V or lists amps just do the same math; amps x 12 (volts) = wattage your PSU is outputting
The term you are looking for is TDP. Search "5870 TDP" or "GTX470 TDP" to find what your looking for.
Edit: Forgot to mention, if your PSU doesn't list the combined output of the 12V rails, you usually CAN'T just add the single rail values together. Look for a statement on the PSU that says "12v1 + 12v2 not to exceed XXXW".