The lenses you were talking about were actually:
AF 28-105 f/2.8-4.0
AF 28-200 f/3.5-5.6
So both lenses open only to their maximum aperture at short end, not when shooting for instance at 100mm.
When reading those lenses on paper I could really guess that the 28-105 is better because of it's shorter zoom ratio, which is always a good thing. But after checking the results from photodo.com
I can say I'm wrong. The 28-105 gets a result 2,4 and the 28-200 gets 2,7. So both lenses are not very good but 28-200 is sharper than 28-105. You should also notice that the 28-80 you already have gets a grade of 2,9 so it's actually sharper than those Sigmas. (5,0 is maximum points)
If I were you I would consider buying Canon's instead of Sigma's. Besides of better optical quality you get the full-time manual USM focusing. EF24-85USM and EF28-105USM are much better lenses than the EF28-80 you have and they are not even much more expensive than the Sigmas. If you feel a need for extra range I would recommend you buy a separate tele-zoom instead of the 28-200. Canon's EF75-300 is for instance very good and cheap lens.
btw. f3.5 is only half a stop slower than the f2.8. For example: 1/125s at f2.8 with 28-105. 1/90s at f3.5 with 28-200.
Hope this helps.