I recently purchased a Digital Rebel XTi for my wife, an aspiring amateur photographer. We bought a body only, then purchased the Tamron 18-200 lens for her primary lens. I'm wanting to build her a good supply of lenses, and recently found a great deal in a pawn shop for a 50mm F/1.8 MkI. I now want to get a good wide-angle lens for the camera, but am a little wary due to some information I've found on the internet about the focal length, smaller sensor, etc. I would have thought that she would get decent wide-angle results with the 18-200 at 18mm, but it isn't enough. I want to stay with prime lenses for their low-light capabilities, and due to my wife's plans to stay mostly with portraiture. Which prime lens would be a good fit for the Rebel as a wide angle? I'm currently eyeballing a 24mm F/2.8. Would this be a good wide-angle lens for the Rebel?
Any suggestions or guidance would be greatly appreciated!