The T900 takes its best photos at and below ISO 200, though some color noise is always present. At ISO 400 things start to get fuzzy and, well, it's all downhill above that. Don't expect great photo quality in low-light without a flash.
While the T900 takes its best photos between ISO 80 and ISO 200, you can see in this 100 percent crop of an ISO 80 photo that there is noise/artifacts even at this setting. Fortunately, you can't really see it at smaller sizes.
Squirrel!!! Though I have no immediate need to print a 13x19 photo of a squirrel, maybe you do. In that case, notice that the 100 percent crop suffers from some mushy detail courtesy of the increased noise reduction at ISO 400.