Hello adelie, and welcome to the Camera Labs forum!
These are exciting times to plan for buying a new camera
Here's my opinion in your case:
- Wait for the successor of the Nikon D200

(no, just kidding

)
- Buying gear with your heart? No, let your head guide you!
- You have a good investment in Canon glass? Go Canon!
- Is there a visible IQ-difference between four/thirds vs FF? You bet!
- Is the risk of blown highlights with landscape photography higher than with wildlife? Yes, unless you shoot birds against the sky!
- Is a 10mm on a croped sensor (equiv. 16mm on film-SLR) wide enough for your landscape ambitions? I guess so. But is certainly cannot beat a 12mm on a FF-body!
- Do you fear that the 5D will let you down on one aspect ore the other? Well, I don't know!
So you're left with three options:
- Buy the 40D, get a decent ultra-wide and be happy now
- Buy a 5D, get the Sigma 12-24 (it's FF) and be very happy
- Wait for the 5D successor, get the Sigma 12-24 afterwards, and be considered a "wise man". Whether you are happy to wait for so long, I don't know
I didn't mention the option for people with v.d.w. (very deep wallets): Wait for the already announced 1Ds mark III
---
In the end it all depends on how dissatisfied you are with your 20D.
Those, who can wait will be rewarded with better, cheaper, more mature gear
Hope that helps...
_________________
Thomas (beware: Nikon-fanboy and moderator!)
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