I have the Canon XTI and use the Quantaray 70-300 lens. During the day it is as good as any but when it is a little dark I can't take photos at all.

What flash can I use to remedy this problem. If it is a flash I would like one that can give me some distance also.
While on vacation I attempted to take outdoor photos of cathedrals but couldn't get the entire object bcause of the closeness. What would you recommend.

I low lights conditions you may experience problems with auto focus system on this lens. I think that's the reason that you cannot make the shot, the camera doesn't find focus point due various motifs: poor optic performance of the lens, poor af drive communication with body processor, etc. I think your problem appears because of poor optic performance. Make a test, with lowest focal (70mm) in low light with you aperture at maximum (F/4 I think). If camera is out of focus, the lens are the reason.
I have read many reviews about those lenses and everyone give them poor rating. My advice: buy Canon, Tamron or Sigma lenses, they have better optics/mechanics - Tamron equivalent is even cheaper than Quantaray.