Hi
I have a 20D, which was purchased to replace a 300D. I love it, and will be keeping it. I still feel it was the camera that showed what semi-pro DSLRs would be capable of delivering.
I shoot landscapes and cityscapes, in all sorts of conditions. I am occasionally called upon to shoot the usual family stuff, but those two are my joy and, um, focus.
My lenses:
17-40L
70-200L
50 1.8
Tamron 28-75 2.8
I am delighted by the Canon lenses, less so by the Tamron.
I like to print to 13 x 19, which is where many 20D limitations occur.
Given my investment in Canon glass, I have first looked there. I was always tempted by a 5D, but decided to put greater effort into my skills and technique and await the next generation.
So now, I have the 5D MKII and the 50D to choose from. The former is expensive, but seems well suited to what I do.
I'm currently a Nikon user myself, but I'm going to suggest that unless there is a really compelling reason to switch (say, you do mostly hi ISO work or are inheriting a rich Nikon using uncles entire lens collection, lol) that you stay on the Canon path given you've started to invest in lenses. I'm a huge believer in building the lens collection first and not worry so much about which body is popular at any given moment.
That being said, a few things to ponder:
a: you would notice the improvement between the 8mp class DSLRs and the 12/15 mp crop body DSLR and definitely the 5d-II FF DSLR, so either option (going to 50D or 5d-II) would take care of that. Obviously going to the 5d-II makes the most sense if you feel resolution constrained with your current system, but it's going to be more expensive. However, with greater resolution comes more demands on your lenses, support system, and shooting discipline.
b: Doing either you also should start to think about some lens realignment. Ditch the Tamron, particularly if you're not happy with it. Going to something like the 5d-II would mean that you could start looking at going down the "best available at each focal length" approach, and that means bypassing something like the 24-105 which frankly isn't of the quality of the best glass out there. Now what I just typed is gonna hack off a lot of 24-105 owners, but hang with me here and think this through - if you're currently unhappy with your 13x19 prints, it could be from a variety of factors including support system deficiencies, lens quality, shooting discipline, post processing discipline to name the common ones. Once you go to a higher rez medium, and that includes the 12/15mp cropped bodies as well as the 21mp 5d-ii, all of those are going to have to be maximized if you really want to get everything out of the body. A 24-105 gets you convenience, but doesn't get you max quality - and mirrors a lot of things already in your range, and part of gaining what you desire at 13x19 is going to be related to lens quality as well as the body upgrade. Now, if convenience is huge with you and allows you to "get the shot", then obviously that's more important than the technical arguments of which lens is better because getting the shot is more important than which lens has xxx lines more resolution than the other, but one also has to be aware of the tradeoffs. So lenses like the 135/2 L or the new 24 L series 2 would be something to strive towards, or, once the Zeiss ZE lineup fleshes out (I don't know which ones are in Canon mount offhand), take a strong look at those if you don't mind manual focus. And if you're doing landscape and not using the very best tripod/ballhead you can get, skip all of the above and invest the money there first before dropping more cash on lenses.
-m