I was just like you when I bought my first DSLR. First thing you should know is that, Nikon specializes in lenses while Canon specializes in cameras. Canon used to be mounted with a Nikkor lens until Nikon made their first camera back in the 40's. Good investment is on the lenses. 2 or 3 years, you would need to replace your camera but your lens will be with you as long as you still loves photography. I could not comment on Canon cameras since I am a Nikon user ever since. If you go with Nikon, I would suggest you choose the D300 especially if you go for sports photography. No other cameras (Canon, Oly, Sony, etc.) offers 51 Area Focus other than D300. D90 is nice, because it has Video capability, but who needs it?

In addition, master yourself on Photoshop CS4 as post processing software.