the Nógrády means "of Nógrád"(originally the noblemen used the surnames in this way from the 18th Century) and the Nógrádi means "from Nógrád". By the Hungarian name-giving tradition in an area/town/country the people got surname by that area/town/ country, where they lived earlier. Usually a name after the place of origin, was given by the receiver community. So, that is the rarest situation, that a person, who lived eg in Nógrád, got name "from Nógrád". This is the cause, why you can find only one Nógrády and only five Nógrádis in the phonebook of Nógrád county, but will find eg 22 Szegedi (szegedi means "from Szeged", what is the capital of county Csongrád, in the other side of the country.... :-) ) By the way, the number of the Nógrádis and Nógrádys in the Hungarian phonebook is near thousand.
Naturally it is an except, the nobleman's prename, what could develop to be a real surname throw the centuries.
Have a good research!
Lajos Reich, Hungary