I have had a 40D for almost a couple of years and not too long after I bought it the 50D was released. Grrr, I thought. I knew what was "wrong" with the 40D as soon as I got it - the LCD is low res and "not enough pixels". But I can tell you that I have had had excellent results from the 40D and if you want to see one taken at high ISO (640) see the following in my gallery...
http://www.photoradar.com/photos/104...s-of-the-choir
It was taken on a 24-105 L lens, handheld leaning against a stone column. Noise? Not much, is there? And any that is there suits the atmosphere of the pic, I reckon. Bear in mind also that this is a quite severe crop from a bigger image so that I could really fill the frame with the choristers. If I'd got any closer I'd have been singing from their hymn sheets!
I've also read that some photographers reckon the 50D is not as good a camera as the 40D. Iin one forum, I read the other day a comment from a guy (in the US, I think) who said that trading in his 40D for a 50D was "the worst mistake" he had ever made!
So,I can thoroughly recommend the 40D and definitely will not be upgrading to a 50D. I can print my pix up to A3 with no problem. The quest for the latest and newest camera model is always tempting and irritating, too. We're all guilty of that. My old 20D has been unused since I got the 40D. Shame... Don't chase the 50D. Go for a 40D - they're quite a bit cheaper now than when I bought mine - and invest in an L lens. Good luck!