"The Greenland ice cap with its volume of 630,000 cubic miles, if melted could yield enough water to maintain the Mississippi river for over 4,700 years." Or: "Volume: 2,600,000 km3"

http://hypertextbook.com/facts/2000/HannaBerenblit.shtml

If Greenland's ice were to melt and if you could somehow dam this water, how much electrical power would this make?

It is guessed that in 2030 the world will consume about 30,000 billion kilowatt hours per year.
http://www.eia.doe.gov/oiaf/ieo/electricity.html

Btw in the US the average yearly electrical use in 2003 was 11,997 kwh per captia.
http://www.energy.ca.gov/electricity/us_percapita_electricity_2003.html
And for some reason California uses least of any state: 6,732 kwh

And will be somewhere around 8 billion people in 2030 so they are allowing around 3750 kwh per year person. So, I suppose it's reasonable.

"In 2005, world electricity consumption was 2,400 kilowatt hours (kWh) per person. India’s was just 432 kWh, four times less than China’s 1,662 kWh."
http://global-themes.com/2008/01/15/of-india-china-apples-and-oranges/

Before I get back Greenland daming. At S.005 per kwh that's 150 billion dollar at wholesales rates, and a retail rates of say $.10 it's a 3 trillion dollar yearly global market. Ok, back to daming.

"POWER (kW) = 5.9 x FLOW x HEAD

In this equation, FLOW is measured in cubic meters per second and HEAD is measured in meters. "
http://www.ecology.com/archived-links/hydroelectric-energy/index.html
Also here:
http://www.jhpl.com/hp-faq.htm

So, let's make Head equal 100 meters [less than 1/2 of Hoover dam]
And Power equal 30,000 billion kilowatt hours per year.
{Which if divide 8760 hours in a year gives average hourly power needed globally of 3.43 billion Kilowatt hours.}

A plant power with a rating of 3.43 million megawatts [Or 3,430 gigawatt] creates 3.43 billion Kilowatt hours per hour of operation and if run constantly creates 30,000 billion kilowatt hours per year [the amount electricity projected to be globally used].

How much yearly flow would be needed?

If the flow were say 58.1356 cubic meter per sec with 100 meter head, the generator would produce 34300 Kwh per hour or be a 34.3 MW generator. So 5,813,560 cubic meter per second gives enough water for a 3430 gigawatt hydropower plant. In one hour it's 20.9 billion cubic meters of water. Or about 21 cubic kilometer per hour. And yearly that's about 183,336.42816 km3 per year.
There is 2,600,000 km3 of ice in Greenland. So, that enough water to last 14.18 years.
If you were to double the height of the Head [200 meters] the water would last twice as long- 28 years [1/2 the water flow- same power].
Three Gorges Dam is more than 100 meters [it's structural height is 175 meters]. Hoover Dam is actually less than I earlier thought: Hydraulic Height: 576 ft It's Structural Height: 726.4 ft
http://www.usbr.gov/dataweb/dams/nv10122.htm

Some dam called Dworshak Dam:
hydraulic height: 632'
Btw, Estimated Cost in 1 July 1973: $302,000,000
http://www.nww.usace.army.mil/html/pub/pertdata/dwapert.htm
Oh, and it was built:
http://en.wikipedia.org/wiki/Dworshak_Dam

Hmm, sort of thought all that ice in Greenland would last longer than 14 years [considering it equals to the flow of Mississippi river for over 4,700 years- or about 500 years if the Amazon River]. I didn't think it was going last thousands of years, but thought it might be somewhere around a hundred or two hundred years.

I wonder if you used all the rainfall that fell on the Earth. How hydropower much that would generate.
Maybe another time.