Hi Eric, welcome to the Cameralabs forums!
Roy's right, Canon supply the same 18-55mm kit lens in 350D / XT, 400D / XTi and 30D kits. It's certainly the weak point in the Canon system, but for day to day shots, it's actually not all that bad. It's only when you start trying to get it to work quickly or under very low light that you begin to notice its budget nature. And it sounds like you'd notice that in your sports events.
The Nikkor kit lenses, whether the 18-55, 18-70 or 18-135mm are all classier than the Canon 18-55mm and we recently compared them here:
http://www.cameralabs.com/reviews/Nikko ... roup_test/
The D80 is also a more powerful camera than the XTi, but you'll be paying extra for features you may not use - and an XT or XTi with the EF-S 17-85mm IS is a better combo than the D80 with the 18-55mm.
As for the XT versus the XTi, if you can get a good saving on the former, then go for it, as there's not a great deal of difference. If the amount is small though, I'd go for the XTi, if only to have better resale value from the more recent model.
Ultimately it sounds like your target photography would seriously benefit from an upgrade in the kit lens, whichever manufacturer or system you end up with.
If you can afford a 30D or D80 with a better lens from day-one (or very soon after), then that'd be my choice. But if it's between a 30D or D80 with a basic 18-55mm kit lens (for some time), or the XT / XTi with a decent lens from day-one, I'd go for the latter.
Don't forget to also check out our 400D kit lens upgrade article to see the differences you can expect with a better model:
http://www.cameralabs.com/features/Cano ... s_upgrade/
Hope that helps!