Canon Lens Selection

[ct_vol]

I am an beginner/amateur photographer at best. Most of the pictures I take are of my saltwater fishtanks, and are therefor taken in somewhat low light conditions... I currently have a Canon Rebel XTi... It came with a 18-55mm lens and a Sigma 70-300mm lens... A few months ago I added the Canon 100mm f/2.8 lens to my collection... I was amazed at the difference of picture quality from my new 100mm lens compared to my old "kit" lenses...

I would now like to add a quality "all around"/walking lens to my collection... I was looking at the 24-105 L f/4 IS, 24-70 L f/2.8, and the 17-55 f/2.8 IS...

My main concerns are that the 24-105 is f/4 and not as good in lower light applications... The 24-70 does not have the IS... And the dust issues with the 17-55...

I'm also thinking I'll upgrade my camera body in the next year or two... I'm confused about this 1.6 form factor I'm not sure what that means, but I don't want to get a lens that isn't compatable with my next camera body, or is limited in its performance...

Any info on these three lenses, pros, cons and opinions would be great... Thanks in advance...

Randy