We were looking at buying a Rebel TXi late last year (its called the 400d out here though) but ended up buying the previous model the 350d instead because they were doing some good deals on the last of the stock.
I've been shooting Canon film cameras for 7 years including some stints as a semi pro photographer during 2001-2002 and lots of motorsport photography for fun.
5 years ago ANY pro photographer would have killed and eaten their own mother to get a camera as good as the current entry level DSLR offerings from Canon and Nikon so whatever you buy I think you will be happy with it.
Anyway a few things to consider
1. The camera is only moderately important in the photo equation. Any of the current DSLRs will do the job and you'd have to be a pro to start to push the limits of the Rebel TXi or the Nikon. I've got one of Canons pro film SLRs but its gathering dust because the 350D is pretty much just as good unless you need to do really fast sports like motorsport
2. The real secret to good photos is good lenses and unfortunately this is where the money is too. $5000 camera with $100 lense = crap photos, $500 camera with $1000 lense = super photos. The lenses supplied with entry level DSLRs are RUBBISH. You might not notice it at web or small print size but you will notice it if you blow up photos or compare the sharpness to pro photos. Canon and nikon both do various levels of lenses and Canon L series lenses dominate sports photography due to their great optics and image stabilisation technology, a feature that inexplicably Nikon has never taken seriously. Higher quality lenses will generally have bigger apertures to allow more light to enter the lense, allowing shooting in lower light conditions. The viewing angle of the lense has got nothing to do with it except that long lenses with big apertures are damn expensive.
So far we've used the Canon 350D and its been brilliant, the nice thing is that it fits all my old lenses and flashes so i didn't have to rebuy everything.
If you want any more advice on the technical side of things feel free to ask