there are certainly some difference between D70 and Canon 20D, but the difference is not like that of between D70 and Canon 300D.
ISO100 vs ISO200 are not very noticable.
70-300 f/4.5-6G is a so-so lens, if budget is not a problem, can consider other lens for that range.
also SB800 flash is a must for indoor shooting.
BTW, Canon and Nikon can both make good cameras, i have seen the fans of these two groups keep arguing about which is the best over the years, certainly i do not wanna see it happens here. as long as we can shoot good pics, who cares what kind of camera u use...
peace!