Take a look at the sticker on the back. It should say how many amps on the 12V rail. I've found the price/performance ratio is much higher with the computer ones than other ones.

This one for example puts out 26A on the 12v rail.

For your charger anything rated at 550W or more should be sufficient. I'm pretty sure that if it has more than one 12V rail you can run the charger off of all of them and not just one.