the opening paragraph of a letter - small or capital letter

How do I start the opening paragraph in a formal letter after having addressed the person - with a small or capital letter? Is there a difference if there is a comma or not after the "Dear X"? And should I consider American/British English?

Thank you!
Radek (radek.majer@nku.cz)

The first word in a new sentence always starts with a capital letter. The opening sentence in a letter is regarded as a new sentence.

In traditional usage, a comma follows the greeting. In the US there is an increasing use of the colon instead.