Itai pretty much answered your question, and Matt Grum and mattdm gave even more good arguments, so not much to add there.
However, I'd like to respond to chuqui's remark that the 50D is a better camera than the 40D. It's not. It's not necessarily a worse camera either. In the Quest For More Megapixels, the 50D sensor pushed beyond what's a physically sensible pixel density, and because of that it's not that much of an improvement. I'll copy the "cons" from the DPReview summary:
- High ISO performance worse than 40D
- Reduced dynamic range in the shadow
areas compared to EOS 40D
- Per-pixel detail not as good as on
good 10 or 12 megapixel cameras
- High-end lenses required to get the
most out of the camera
- Live view not as accurate as on 40D
(framing very slightly off-center, in
contrast detect AF mode not possible
to magnify right out to the extreme
A low-light photographer like me, who prefers a high dynamic range, good high ISO performance and good Live View (it's a great way to reduce camera shake and focus manually in the dark), prefers the 40D over the 50D. If you're more of a studio photographer it's a different story.
However, the 50D has another thing working against it: because of the ridiculously high pixel density, you need really, really good lenses to actually take good advantage of the 50D's extra megapixels (the linked summary explains it rather nicely). If you don't have good glass, it's not really worth the upgrade.
In the same sense the 5D's age can actually be an advantage: because it has relatively few megapixels (12.8) on a full frame sensor it has a very low pixel density. That's the main reason it has such great tonal range, contrast, etc. It also means you don't require lenses with insane optical sharpness to make good use of the camera. Oh, and the RAW files aren't as huge.
So given the kind of photography you do, and the fact that you don't have any L-lenses, I think the 5D has the advantage in this aspect as well.