Well you shouldn't be getting poor quality results with the 70-200 f/4, it's a pro quality lens. Granted I shoot Nikon, but I shoot with the 70-200 f/2.8 VR which is basically identical to the 70-200 f/2.8 IS from Canon. The Digital Rebel shouldn't be giving you that poor of results either, especially coupled with the pro lens you have.
Where do you do the majority of your shooting with the 70-200? The only gain you'll see in the 70-200 f/2.8 IS lens is in indoor or low-light handheld shooting. Otherwise past f/5.6 they will both give you the same basic image quality. Even wide open at f/2.8 your image quality won't be that bad unless you're over scrutinizing your work. I've got beautiful shots at f/2.8 with mine.
Any way you can send some pics my way? I'm curious to see what you mean by bad quality. Shoot me a message or use my email if it's in my profile (I just joined this site so I have no idea how profiles work).
If you're not doing much low-light handheld shooting, than there is no point in upgrading to the f/2.8 IS lens, as you won't often use the f/2.8 aperture or IS. I know people rave about the Canon 70-200 f/4 lenses because of their small size and light weight, whereas you'll be carrying a much bulkier and heavier lens with the f/2.8 IS. And if you're comparing between the 50d and the 5d...well it depends on what you're shooting, but I'd jump for the 5d mark I. Since the new 5d Mark II is basically out, you can pick up a used 5d for a real steal. FF 5d will blow away the Digital Rebel you've got and perform better than the 50d. But before you upgrade anything, it would be great if we could see some of the photos that you're not happy with so we know what the problem is.