Replace my Canon 28-105 with what ?

Now that I've gone digital I'm looking to replace my Canon 28-105 for something sharper and wider. I'm currently tempted by the Tamron 17-50 but also think I might like to go even wider say a Tokina 12-24. The question is do I get the 17-50 now and then if I want to go wider effectively "waste" the overlap by something like a 12-24 or bite the bullet and get a sharper 28- something e.g the Tamron and live without the wider view until I can pair it with an UWA.

I'm looking to use thes lenses for garden and landscape photography. I know that I'd love Canon's own glass having just stumped up for a 70/200f4L but unfortunately finances will only allow an independant lens purchase

All thoughts welcomed
Andrew

All I can say is that the Tamron 17-50 is an amazing lens. I have 2 L's and am about to pick up a 3rd and I will keep the 17-50 so long as I have a crop camera. I even have a 10-20, but still love the Tamy.

I replaced mine with the 18-55mm IS. It's much wider and I've found it to be much sharper (even wide open). The IS is a nice bonus as well. It's a bargain at around $100 used.