Nearly all lenses are sharp at f8 and f11, and you can stitch multiple pictures together if you want to go wider.

If landscapes are your thing, then what you need much more than a 5D, or a wide lens, is a totally bomb proof tripod.

Use the money on a Manfrotto 055, or similarly solid tripod, and you wont regret it.

While I find personally I dont get on with nikons as much as Canon's, I really don't think going from a d80 to 5d would make much difference at all. Certainly not as much as making sure your pictures are sharp by using ISO 100, f11, and having a solid tripod to attach your camera to