Dont buy any clothes here on the street unless it's from a yard sale, they're almost exclusively fake.
For shopping, you can go ANYWHERE pretty much but there are two major concentrations of shopping and they're both REALLY big. The area bounded by the bowery on the east, broadway on the west, 14th street on the north, and canal on the south houses a HUMUNGOUS amount of shopping: Barneys, 3 diesel stores, j lidheberg, a levis store, lounge, atrium, blue and green, cloak, replay, evisu, bape, APC, puma, bloomingdales, gstar, and about a thousand other places are all within that area.
Fifth avenue from about 42nd to 59th as well as lexington in the same area and all the sidestreets are full of all kinds of really upscale shopping - Dior, D&G, Escada, Saks 5th avenue, barneys, bloomingdales, and all kinds of super expensive little boutiques are in there.
There's lots more but if you're here for a weekend I would just focus on the soho area. Check the superfuture map - it's a little out of date but it shows you the general area involved and how much shopping there is:superfuture :: new york :: soho :: mapsuperfuture :: new york :: nolita :: mapsuperfuture :: new york :: lower east :: mapsuperfuture :: new york :: west village :: map
There's even some stuff that's not on those maps too...