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If you think you need full frame and are looking at 5D-series bodies, I would not get the 5D unless you are unable to afford anything better. It was a fine camera and still is, but there was a significant change between the 5D and the 5D2: 12MP to 21MP, live view added, better high ISO performance, effective dust reduction system (important!), video, and so forth. If you are looking at a used camera and can afford a bit more, I recommend that 5D2. Note that 5D2 prices have been dropping quickly recently, following the introduction of the newer 5D3.
The 5D3 is an improvement over the 5D2, though the differences, while real, are not nearly of the magnitude of those seen when comparing the 5D and the 5D2. If you have the money and aren't worried about cost and want a new camera, that can be fine, too.
The two common standard zooms on Canon full frame cameras tend to be the EF 24-105mm f/4 L IS and the 24-70mm f/2.8 L. A newer version of the 24-70 is coming soon - reportedly better in some ways but also very expensive. Both lenses are excellent, and both have their strong fans and their detractors. I finally got tired of responding to the questions about comparing the two, and I posted a sort of summary at my blog: http://www.gdanmitchell.com/2011/02/06/canon-ef-24-70mm-f2-8-l-versus-canon-ef-24-105mm-f5-l-is
The 17-40mm f/4 L is a fine lens and one of the two most common ultra-wide Canon zooms on full frame cameras. The other option is the 16-35mm f/2.8 L. (The newer version II has slightly improved performance at f/2.8 by comparison to the previous version of the lens, but it also requires the use of a larger and slightly non-standard 82mm filter thread.)
If you are primarily going to use such a lens for tripod-based, small aperture work - say landscape or architecture, etc - the 17-40 is an outstanding performer stopped down. It is also smaller, lighter, and less expensive, and it is resistant to flare. If your primary intended use is something like handheld, ultra-side, low light photography, then the 16-35 may well be your choice.
Your question brings up a couple of other issues. The first might be the cropped sensor versus full frame question, and this may also play into your cost considerations. Current Canon and Nikon cropped sensor bodies can produce truly excellent image quality. The primary advantage (though not quite the only one) of the full-frame bodies is the potential for somewhat better resolution when producing very large and very carefully made prints. If you are not going to be regularly pushing the upper boundaries of print size (say, making a lot of prints at 20" x 30" and larger) then you might seriously consider a cropped sensor body. You may be able to get excellent quality at a lower cost, and get more/better features on the less expensive cameras.
The other issue is the question of why you would switch to Canon from Nikon. I suppose that having bought a lens, you are now committed. However, I have to point out that brand switching is rarely the panacea that many imagine it will be. Frankly, both Canon and Nikon (plus others like Sony and Pentax...) make excellent cameras and lenses. It is impossible to tell which brand was used by looking at photographs, and excellent photographers choose both brands. While I'm a Canon user, in a way that is more a result of chance than anything else, and I'm certain that if I shot Nikon I would produce much the same quality of photographs that I produce now. Basically, it is typically better to stick with a brand...
Edited on Sep 03, 2012 at 09:02 PM · View previous versions