Upload & Sell: Off
If IQ is your main objective, and you are shooting 1D3 bodies currently, I can't think of any reason to get a 40D/50D unless you need a backup camera and have but a few $$$ to spare.
I posted this topic a couple of months ago, you might be interested in reading it. I compared my - then - current cameras 5D and 40D to a newly acquired 1Ds2. I only tested high ISO noise performance.
Before you click on the link, please read the following. Despite a clear introduction of how I tested (no 100% crops), there was a lot of confusion by people who didn't read and just looked at the images. So:
Please take notice of the fact that I did not show 100% crops. I find 100% crops are useless because nobody puts a 100% crop in a wedding album. When you compare 3 cameras using 100% crops you know camera A is better than C and C is better than B. You can't tell if cameras B or C might be good enough for your needs. Therefore I shot the 3 cameras, applied my workflow, and exported to JPEGs in such a way that the resolution you see on screen is the same as the resolution you would see in a wedding album. You should position your eye at the same distance as you would view an album.
OK now you can follow the link.
In my test, the 5D is the best, the 40D the worst and the 1Ds2 comes at a close 2nd. ISO performance that is. There are other factors that set apart the 1Ds2 and the 5D that I did not test here.
As for 40D vs 50D: there are tens of topic here on FM. I read them all before I bought my 40D. I value IQ more than anything and there seemed to be concensus on the 40D beating the 50D at high ISO noise, but still most say they'd prefer the 50D for other reasons, like MA, better menus and such.
What did I decide?
Eventually I sent back the 1Ds2 because of AF problems. I sold the 40D and bought a used 5DmkII. So I'm now shooting 5D/5DmkII combo. I'm quite happy with that now and I won't need another camera for the next 2 years, unless I move into weddings more, in case of which I'll get a 5D3 probably.
Hope this helps.