Buying Advice

[Arggg]

I really need some advice here..
I am currently 16 and have been getting into digital photography. I have mainly been taking photos of local skateboarders, that's what I do. Right now I have a Canon EOS Rebel XT. I recently broke the standard lens that came with the camera due to a flying skateboard >.<. Im not really happy with the rebel xt and was looking at a Nikon d90... orrrr instead of a d90 I was going to just replace the broken lens with a Canon EF-S 17-85mm f/4-5.6 IS USM, about $430 here, Canon | EF-S 17-85mm f/4-5.6 IS USM Autofocus | 9517A002BAWB . Soo, for an amateur photographer doing what I do... What would you suggest? Also, if not one of these, what do you suggest I do? and, Is there a somewhat equivalent lens to the one above that would fit on the d90 that I could get also with the d90? Give me suggestions. I have about $900 but would rather not spend it all...
Throw out your best.
Anything helps.
Thanks.