I have thought out a fairly novel way of getting the actual game mechanics down before writing a single line of code. Basically, I work out the rules, and a set of trading cards, print them out, and play the game manually with a few friends to test out the idear. This should allow plenty of potential problems with the actual gameplay to be realized before they become coded into the game.
Realizing that this could be an interesting way of generating profits, having people purchase booster decks and whatnot online, I have decided (for the time being at least) to avoid doing this. The main reason being that I don't want to have to manage a server that store's all of this information, or deal with people who have purchased these things online, or worry about any of the legal implications involved. For now, I just want to make the game.
You might also go to the business-card-printing department and ask if they have any misprints. If you're lucky you'll score a couple-thousand misprinted business cards. Perfect for prototype game-cards. Easier to shuffle than index cards.