I'm not from USA so I won't answer your question directly (other posters are better for that). Instead I'll point out a few things, based on knowledge of Linguistics plus other stuff:

• Cultural differences are somewhat objective, but how you split the cultures is subjective. As such don't be surprised if different maps show different divisions.
• Language usually play a huge role, and isoglosses are often used to demarcate cultures. However, people shouldn't confuse dialect+language with culture, as it's perfectly possible to lump together two populations of different dialects as the same culture, or to split a single dialect into multiple cultures. Also, dialects themselves tend to be subjective, like the above.
• Geography does play a role too, but it boils down to interaction between settlements and identity. For example you're more likely to contact the guy in the other city (thus share the same culture) if there's just plains between your cities, instead of a big fucking mountain.
• A good place to look at the cultural divisions is the original settlements. Based on that I think that it'll be easier to demarcate cultures in Eastern USA than in the West.
• More often than not cultures don't give a fuck about government borders. So don't be surprised if some cultures grab "chunks" of USA plus either Mexico or Canada.