Quality upgrade for Canon 50D?

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Hi everybody. My name is Rob and I'm saying hi here as it's my first post on the forum.

Now for the question. I have Canon 50D at the moment and recently purchased a second-hand Canon 17-40mm L lens. As brilliant as it is (it brings whole load of quality to the pictures), I feel that it exceeds what 50D can cope with. I'm talking especially about poor noise reduction and wanting to get more details and general quality.

I now have some money to spend. I can't really afford 5d Mark II, but I was thinking about Mark I, as you can get one for a very decent price nowadays.

I was hoping to hear from you, what are the differences between 50D and 5D MarkI in terms of quality, detail, sharpness and noise control. I mainly shoot urban, landscapes, portrait and some product photography. I'm not bothered about pixel count, as my work doesn't get printed and is displayed digitally.

Thank you for honest and comprehensive answers.