DPI is only meaningful for hardcopy (prints). 512x512@1000dpi is the same PIXEL resolution as 512x512@1dpi. As a standard rule o' thumb, the texture map should be as large as or larger than the pixel resolution of the rendered image or area of the image that the textured object covers. If all else fails, double the texture map size and see if that removes pixelization. Then, take half of the difference (4000-2000 = 3000) and see if the pixelization is still there or not. Continue ad infinitum (or until you are satisfied!) :)