http://spacecollege.org/isee3/isee-3-reboot-amplifier-instal...
It is an AR 700S1G4 with rated minimum output power of 700W, and will be used in S-band. Here's the datasheet:
http://www.arww-rfmicro.com/post/700S1G4.pdf
At these frequencies, the Arecibo dish has a gain of around 72dBi, putting the EIRP at about 7 gigawatts.
For scale, a typical wifi router has an EIRP of around 50 milliwatts, so this is about 140 billion times more powerful.