Hard to tell your exact situation. Some depends on the market you are serving (is it regulatory-based, Web 2.0, indirect channel driven, etc.), the product you have and it's maturity, and other things.
Personally, I don't think you need a large board at this stage. One insider (you), one outside board member who brings expertise, perspective, or contacts you are missing, and your investors. Three investors would appear to dominate, so it might be good to get a second outsider, if there is some value add that could be defined. Ideally, you'd get down to 2 investors and reduce the "martini effect" - "one martini feels good, two feels better, and three guarantees a headache". But this can usually only be done (carefully) during a new round of financing.
As to frequency, a lot depends on how much the board trusts you, and what their role is. If you are at an early stage where most of the time is exchanging ideas and advice, there is benefit to higher frequency. If you are at the "monitoring" stage, then once a quarter after the quarter has ended is typical. A common compromise is to meet bi-monthly. But the board meeting should just be a formal complement to frequent communication between you personally and each board member.
You might find this site useful as well (don't know if it's kosher to ExpertCEO to post another link, but here goes):
founderresearch.blogspot.com/2005/09/bui...ship-monitoring.html