The reality of selling ad space is a tough one, most smaller sites are not appealing to larger brands. you could add ebay auctions for game related items. Sure they dont have credit cards but their parents do.
If the idea of your age demographic being able to make a purchase concerns you try CPA offers. A win a free xbox offer where they enter their email and complete the path. Or you can find any one of the new social networks popping up like perfspot or takkle who are in CPA networks paying per registration.
A key point to realize is if you add PPC you're adding another affiliates links on your site most of the time who in turn is taking your visitor sending it to a CPA offer and making enough to pay you, and search engine and keep a profit. I would suggest putting adsense up, watching the ads (let google tell you what offers work best) and clicking them to see where the links go and if you can find the same deal in a CPA network run it yourself, you should double your earnings in theory by cutting out the engine. It is a bit more work but another way to squeeze more juice out of the sponge.
Just my

most people might disagree with me and be right about it.