New York galleries are putting on their best outfits this month for the Europeans in town for the Armory Show. Perhaps the single most inspiring block in the city at the moment is 21st St., which currently has several remarkable shows, all sculptural:  Robert Grosvenor's neo-primitivist vehicular constructions at Paula Cooper Gallery, Alexander Calder in a spare, pristine, iconic installation at Gagosian, Banks Violette's tripartite billboard at Barbara Galdstone, and at 303, Mike Nelson's Quiver of Arrows (detail above), a seedy habitation comprised of four conjoined travel trailers, furnished with the trappings of Pakistani/Afghani-British immigrants, much in the manner of Jonah Freeman's Black Acid Coop. Known to New York largely for his 2007 Creative Time installation, A Psychic Vacuum, an architecturally-scaled navigable partial-readymade on the edge of Chinatown, Nelson's new work is smaller, tighter, and more thematically focused than the last piece.