Help choosing lenses

From memory, it'll work, but it won't autofocus. A 2X teleconvertor knocks 2 stops off the lens, which takes it from F4 to F8 (see details here). I think the 20D/XT need a minumum of F5.6 for AF to work properly. Someone will correct me if i'm wrong.

thats fine but what about these extension tubes how well do they work?

Dunno, never tried, them, reports are they work well. You'd be manually focusing with those things on anyway, so lack of AF wouldn't much matter.

Yes the 2x will lost 2 stop but the 1.4x will only lost 1 stop.
Yes f/5.6 is the minimum. Look at the BH link you provided it in at the botom of description.
There is no lens in the extention tube so there should not be any light lost.

well i got my sigma 70-300 and my canon 100mm this morning and im very pleased, i will post some photos when i get back from nicaragua. thanks again for all the help.