The so-called `TV white spaces' (TVWS) - representing unused TV channels in any given location as the result of the transition to digital broadcasting - designated by U.S. Federal Communications Commission (FCC) for unlicensed use presents significant new opportunities within the context of emerging 4G networks for developing new wireless access technologies that meet the goals of the US National Broadband Plan (notably true broadband access for an increasing fraction of the population). There are multiple challenges in realizing this goal; the most fundamental being the fact that the available WS capacity is currently not accurately known, since it depends on a multiplicity of factors - including system parameters of existing incumbents (broadcasters), propagation characteristics of local terrain as well as FCC rules. In this paper, we explore the capacity of white space networks by developing a detailed model that includes all the major variables, and is cognizant of FCC regulations that provide constraints on incumbent protection. Real terrain information and propagation models for the primary broadcaster and adjacent channel interference from TV transmitters are included to estimate their impact on achievable WS capacity. The model is later used to explore various trade-offs between network capacity and system parameters and suggest possible amendments to FCC's incumbent protection rules in the favor of furthering white space capacity.