OS/IS is a great idea for telephotos, but does add to weight, size and cost. It depends on priorities.
I have the Tamron 18-200 which is extremely useful as a 'travel' lens and for situations where I don't want to do a lot of lens-swapping because of dust etc. It complements, rather than replaces, other lenses and is used in different circumstances. If you have the same requirement, an all-in-one lens like the Tamron or Sigma could be just the ticket, but they do have limitations in terms of maximum aperture and absolute image quality.
Are you happy with the kit lens? The quality of these seems to vary, and some examples are better than others. If you have a good one, it may be better to add a super-wide (Sigma 10-20, Canon 10-22, etc) which is good for interiors, landscapes and architectural shots.
Your 75-300 isn't the very best of lenses, but the newer Canon 70-300 IS is extremely good IMO. That would be on my shopping list as a replacement.
If you'd like to replace the kit lens, in addition to Dan's suggestions you could look at the Canon 17-40L which is good value and will also suit full-frame if ever you go for that.
HTH.
