With the 100 leds.. do you want to be able to turn on and off each LED individually? Do you want to be able to fade them in and out?
Think you need to do a bit more research on what you want to do, and also some of the Arduino Examples (even before buying, to understand em.) If you just want to turn on 100 LEDs when you push a button, you just need 100 LEDs, their resistors.. a button, and that's it.
If you want them to stay on after you push the button, could use ... alot of different choices, but an Arduino included. The only time you'll need the ATmega1280 (Arduino MEGA) would be... well, only if you need some extra analog inputs, outputs, and don't feel like buying extra components. (The Duemilanove / 2009 can handle 100 LEDs with just a few extra chips, the HC595's will do)