The Sigma 10-20 is great, I've had mine for a couple of years now and its never off my camera for long.  The F4.5-5.6 is supposed to be slightly better than the F3.5 but if nothing else its also cheaper.

If your wanting to do much in the way of landscapes with it you'll probably want to think about some filters, I've got a set of Hitech soft ND Grads which were only around £30 but if I start doing some more landscape work, which I want to, then I'm probably going to have to get a set of Lee ND Grads as I can only have one on at a time at the mo with the HiTechs.  Just something else to bear in mind as Lee's aren't cheap.

As for trading in the 17-50, I'd say it depends on what you'd then use as a standard/fast lens after that, you'll not want to use the 10-20 for everything so 17-50 is still quite a useful lens to have really, in fact I'll be buying one this year hopefully.

You might not be too bad just using the 24-105 then, the 10-20 will cover you up to 20 so you've only got a small gap, just the f2.8 you'll loose really.  Not sure what I'd prefer, you can make do without the 17-50 range quite easily but then the F2.8 is rather handy if for nothing else than the odd time your doing low light and the 50 is too long.

I'd say it comes down to what you shoot most of at the mo and what you want to shoot more of, if you want to shoot more landscapes then you'll want the 10-20 for sure.

I've heard the tamron's AF is slow and loud too, the Sigma 17-50 is meant to be nearly as sharp but has a faster focusing shame its more expensive, might have to get saving my pennies.

Right don't let the noisy 17-50 Tamron put you off, it's supposed to be better than the Sigma in that range (IQ wise), but I'm probably going to get rid of it for the Canon alternative, which is about twice the price and not an L lens but the glass pretty much is, but is a better lens (I wish they did an L version TBH).

However my Tamron 10 - 24mm is a really nice lens, great reviews and doesn't sound like Jonny 5's eyebrows like the 17-50! Check the comparisons before going for the Sigma And if you are still concerned then make a trip down to Jessops and have a play, it's a much different lens to the 17-50

As for getting rid of something in the 17-50 range, ok you may have lenses coving that range but around the cross over point you will probably end up taking a lot of pictures at  each side, which isn't great if you need to keep swapping lenses..