Between ISO 1600 and 3200, quality takes a little dip. And from then on, the dip becomes deeper. But for me 3200 is still usable, noise is okay. ISO 6400 for emergencies only with noise-reduction applied in post. Depending on your processing effore, you might be able to pull off 6400 with 8x10 prints... But to be on the safe side and not lose too much image quality, try to stick with 3200.