Digital cameras allow you to adjust their "film" speed, or ISO level, dynamically, so you can always tweak your settings depending on how dark your environment is. But camera technology has advanced quickly and significantly in the last few years and while the rule of thumb with ISO levels has always been "the lower the better", does this hold true with today's advanced sensors, processing chips and software?
As Adorama TV's Mark Wallace explains, it does for the most part, but what you can get away with has increased dramatically. Upping the ISO to 400, 800 or even 1600 isn't so bad, seeing as cameras now support numbers in the 6400-12,800 range (and even 409,600 for high-end models).
That said, different cameras have different sensors and while some can hit extremely high ISOs without generating too much noise, the quality variances can be extreme between manufacturers, especially if your DLSR isn't the newest hardware on the block.
If you haven't already, Wallace recommends spending some alone-time in a dark room with your camera (not as dirty as it sounds) and taking photos at increasing ISO levels. Then download the images and check them out in your image viewer of choice, looking at the amount of noise introduced with each level.
You should be able to find a good compromise between quality and lighting -- a good fact to know before you start taking proper photos.