If you do have to use a high ISO, e.g. 1600 or 3200, then you'll get noisy pictures. The Canon SLRs have a (very well deserved) reputation for producing clean images even at high ISOs, however at 3200, this noise is definitely an issue with any camera.
Fortunately, there is a wonderful piece of software called "Noise Ninja" which does a fantastic job of reducing this noise..... just google for it.
Mik, aka PPD