I check out your site a couple of times now. Personally, I would still work on the design a bit. Make it easy to navigat, read and consume the presenter info. To attract advertisers you need to make sure you reserve spot for the ads that are of interest to your future customer.
Apart from that you could start right away. Why not? You alredy have more visitors per week than an awful lot of other blogs and website. You are not forcing your customers into a deal. All you need to do is find someone how believes in your project and finds it is worth the investment. As you said yourself, it is not a huge inversment we are talking about.
Think about starting off with a fire sale of ad spot.
Inspired Money Maker started a firesale last week. I never thought about placing an ad at his site. Still reading his post about the sale I thought: Well, why not? I would have spent that $5 anyway.
That is how he got me to advertise.
Also think about using such an automated ad tool for your site at the beginning. Let me know what you do - I might be your first customer!
Hope this helps,
PhilK