I generally render mine at 1500x1500, 300 dpi. When I save them, I save as PSD, or bmp. Jpeg'ing it usually ends up with the wonky lines around the form, like what you have in your image up there. The way I do it takes up a hell of a lot of space, huge files and whatnot, but the quality is rather good once I change it back to Jpeg after finishing it up in Photoshop.