I bought my first DSLR in 2005, and was choosing between a 20D and a Nikon D70. At that time, Canon's CMOS sensor tech was superior to Nikon's CCD tech, especially in low-light shooting. It was impressive that the 20D gave a "clean" ISO 400 and a "usable" ISO 800, as I recall.
There were other things I liked better about the 20D and the Canon system (AF, base ISO of 100, 8mp vs. 6mp), but a camera dealer in Wichita, KS, said he had more Nikons come in for repair than Canons. That sort of helped seal the deal at the time.