Hi
Modern digital cameras allow you to 'push' the ISO setting with very little deterioration in image quality - normally seen as noise sometimes called 'grain' on your images.
If you have previously set your camera ISO to a setting that would maximise image quality eg. ISO 100 then you could push the setting to 800 or in some cases 1600 for indoor images. Some advanced bodies allow you to push the ISO beyond 3200 without much image deterioration. What you will benefit from is to add definition to low light images by using a Speedlight flash unit. Look at the Canon 430EX or if you can afford it a Canon 580EXII. Second hand 430EX or 550EX units may also be worth considering if tested fully before buying. Alternatively add a sturdy tripod to your kit if your subjects are stationary. Acquire a cable release or master your on camera shutter delay function so that you aren't reintroducing camera shake into your tripod shots by triggering the shutter by hand.
Good luck
Adam
www.adamcoupe.com/portfolio