Nice stuff. Look into getting a Wireless Keyboard and Mouse too if at all possible. I hesitated when I bought my iMac and I regret it because I ended up buying them separately. It's almost impossible to find a good home for a wired keyboard.
Def. get the 20 inch display. It's great having that much screen real estate. The 17 inch is great too, but the 20 inch has more of the "wow" factor. Whichever one you go with though you'll be happy with. Enjoy your new mac!