Congratulations on the 20D. I am still using mine and it is a great tool!
it depends all on what you want to shoot. Ok you want:
- a good all around lens
- that gets wide but
- also has a bit of a zoom to it
- Something I can shoot architecture and landscapes with,
- as well as the occasional portraits.
I would not buy one lens for that. Wide (on a 1.6 crop body) + portrait does not work very well. You will need 17-85 or a even larger range. These lenses are not necessarily the best (optical) quality.
The best option might be a Sigma 17-70mm F2.8-4.5 DC MACRO. Check the reviews. I like the 24-105 F4L, but it is not very wide on a 20D and also costs 1,000$.
Below are a few of my personal (!) preferences/ thoughts on lens selection.
Cheers.
1) Nowadays I would not buy a lens anymore, which does not work on a full frame (FF) body. Like the Canon EF-S lenses. A good lens you can keep a long time. And sooner or later you might want to upgrade to FF.
People say good things about the 17-50 2.8 Tamron. But AFAIK this one does not work on FF?!
1000$ for a 17-55 EF-S? Not for me, thanks.
2) In general it depends all on what you want to shoot and your preferences: zoom (=convenience) vs. prime (=better image quality/ better low light performance, lower price).
With three Canon primes you can get excellent optical quality: 35mm F2, 50mm 1.8 and a 85mm 1.8. Built quality is similar to the 50mm you have. But hey, these are between 80$ and 330$.
However, if you do microstock (MS) semi-serious. You will be able to afford pretty much any gear from the earnings.
3) Also I would not buy medium or low quality lenses. Sooner or later you are going to sell them to get something better. And if you do MS you will need good quality lenses.
I am not a fan of the Canon 17-85. I used it and was not very happy. I did not have/ use the Tamron 17-50 2.8 or the Sigma 17-70, but from what I read, these are better lenses. Again, all not FF.
4) For shooting MS I would recommend a macro lens (for objects etc) eg Tamron 90mm 2.8. Can be used for portraits too. Check the reviews and compare with the sigma 105mm and Canon 100mm macro.
And a 24-105 F4L. I find this lens very convenient. For MS most of the stuff I shoot is >F8. However, if you want to shoot sports (espec. indoors), wedding, kids at home etc., this lens would not be your best choice.