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.
The reason I don't want to go with the resistors and a 555 timer for delay is because I want more complex lighting effects such as a breath effect (diming) and in the future implement some more complex lighting such as Christmas tree style flashing and commands. These are just some ideas that I want to do to learn.
Between Arduino Mega and Duemilanove, if they are the same price (which I don't think so) buy MEGA which has more ram and I/O than Duemilanove.
Arduino Mega $27aud http://goo.gl/v7x2P
Arduino Duemilanove $26aud http://goo.gl/J8sQ4
I want to keep it as simple as possible, at first I just want them to come on for x amount of time when a trigger is activated so if that is possible from the single I/o pin that that is great.
I am asking this with little research because I am asking for one as present for Christmas and need to get my act together and say what i want ;).