It doesn't really matter what body you're using. The rebel series take great pictures, some of my best are from my XSi.
Granted, you'll see a much bigger difference if you're using a 50mm 1.2L on a 5D Mk II than compared to on a XTi. But that's a case of full frame vs crop.
Generally, it's a great idea to upgrade glass first. Unless it really is the body that isn't doing what you want.
If you're trying to get a fast burst rate and solid autofocus, maybe the 7D is what you're looking for.
If you're trying to get crazy shallow depth of field and high resolution, maybe the 5D is what you're looking for.
For anything else (maybe you don't necessarily need any of those things but want them just a little bit), the rebels will do great. I've shot plenty of snowsports on my rebel, as well as portraits and landscapes. The rebels are a great camera and with some great glass in front, they'll perform marvelously.