What telephoto lens (400mm/500mm) will a Canon EOS Rebel XT support for the use of taking sports pics (football).
There are no limitations on the lenses you can mount on your Rebel, except of course the Canon manual focus lenses.
There is at least one manual focus lens made for both Canon and Nikon Dslrs, by an independent maker.
ars longa vita brevis
What will be the noticeable difference in pics with a f2.8 vs an f5.6
If you're shooting apertures 5.6 or above, the only difference will be the weight of the lens. An f2.8 lens requires wider diameter glass, so it will be heavier.
You might want to pick up a basic photography book to learn more about cameras and exposure.
I shoot. I write.
If you let us know what lenses you are considering, and what images you want to capture along with the conditions you expect to encounter, we can give you much better answers to your questions and/or advice about what lens(s) will be most suitable for your use.
There are no rules for good photographs, there are only good photographs. -Ansel Adams
A lot also depends on the size of the prints you're making. Almost any modern zoom lens can make very good to excellent prints up to 8x10. If you don't crop a lot or make larger prints the difference would barely be noticable even in a side by side comparison. The same applies if your images are strickly electronic.
The EOS350D has a maximum ISO of ISO1600, and IMO a “quality usable ISO” of ISO800.
Considering daytime football under moderate/light cloud cover, the typical EV (Exposure Value) of the scene will be around EV = 12~13.
For transverse subject motion (subject moving across the lens’s axis), typically, for amateur football played by youths 15years, we would require a Minimum Tv (Shutter Speed) of 1/800s.
At EV12 and ISO800 to make a Shutter Speed of 1/800s we require an aperture of about Av = F/5.6
Therefore, if the cloud cover becomes “moderate/heavy” or “heavy” and the lighting drops below EV=12, you only have one stop of ISO left in the bank to bump.
Any darker and many shots will become “impossible” for an F/5.6 lens and that particular camera for that particular task.
Under typical Stadium Lights (for non televised or amateur games) it most likely will be impossible for any shots with an F/5.6 lens.