Nikon is replacing the D40 as its least expensive digital SLR in favor of the Nikon D3000, a $600 camera that raises resolution to a more respectable 10 megapixels, the burst rate from 2.5 to 3 shots per second, modernizes the autofocus system, offers a sensor-shaking dust reduction system and comes with an image-stabilized lens. The question, though, is whether even with all these improvements, the D3000 competes effectively in what’s now a highly competitive entry-level SLR market.