I'm comfortable with cost estimates, budgets, and investment criteria, but I am not an economist. So with that said, here is my advice:
1. Make sure that your store has a website, where customers can reserve books and come in to pick them up, or have them shipped to their door. This will allow you to reach more customers anywhere in the world without spending much more overhead (advertising, for example.) A good site can cost you a couple grand (or more), so save money by building and hosting the site yourself if you have the skill.
2. It is very difficult to run a successful business on a single product, especially a product with such a specific niche as roleplaying games. Think of other products or services you could offer that would compliment your business plan and expand your customer base...used books, models and hobby supplies, video games, comic books, and so forth are popular choices, but even a hot dog cart in the right area could be highly effective for supplemental revenue. In fact...
2a. If you are going to host games, tournaments, and other games at your establishment, you should consider putting an espresso machine at the counter. Coffee and fountain drinks have low initial costs that can be sold for high return (and gamers need their caffeine.) If you get a good enough of a location with a lot of foot traffic, the espresso bar could almost generate enough revenue on morning coffee sales alone to carry the rent, allowing you to sell books and gaming materials at break-even profits (which means you can stay competitive with online book retailers.)
3. Listen to Mistwell, above, because that is some really good advice for any business...not just gaming.
Good luck! I would love to have my own gaming store, but I'm too much of a chicken to open my own. Besides, it is almost impossible for a little guy like me to compete with
our local used bookstore.