Before everyone jumps in trying to tout their favorite lenses, let me suggest you might give a few more specifics about what exactly you're trying to achieve with the upgrade. Because you already have the range covered... what do you shoot? In what way(s) have you found your current lenses to be inadequate? What do you want the new ones to do that your old ones can't (or don't do as well)?
Whenever anyone mentions cheap and IQ in the same sentence the Tamrons come into the equation
I would consider the 17-50 Tamron and use legs to cover the gap between 50-70mm
My personal choices would be the Tamron 17-50 and a S/H sigma 70-200 2.8, but from what I have heard here the Canon 70-200 F4 also comes highly recommended in the place of the Sigma unless you abslutely "have" to have the 300mm Focal length.
I'm not sure autofocus will work with the teleconverter either.
I have the 70-300 IS you've asked about and it has produced some nice images for me and the IS works very well to improve the usability of the lens. The reviews that I've read have consistenetly shown that it produces better images than the 75-300 series of lenses. My images have been consistantantly better than 75-300 samples I've seen in the past and are better than my old 80-200 Pentax consumer lens ever produced.
I've read consistently good reviews of the Sigma 17-70 2.8-4. Tamron has stolen a bit of the spotlight with their 17-50 2.8, but the 17-70 sample shots I've seen have had good color, contrast and sharpness. I'd probably try each out and see what works best for you if you can find them locally.
I wouldn't say that the focal lengths need to match up though. In my kit I have lenses that overlap signifcantly and places where there are gaps. Well, I don't really have any gaps if I carry all my lenses, but I often leave some of my lenses at home when I backpack or travel. Say I've got a gap from 50 to 70mm. I can usually address the distance to subject to resolve any composition issues in this range. It isn't a common landscape photography range on a crop camera (where framing sometimes can't be changed by moving).
