I upgraded from a 450D to a 50D a couple of weeks ago, and I'm so glad I did. My main reason for doing so was the superior ergonomics and added flexibility of the 50D, I wasn't really expecting an amazing improvement in the IQ. Also, I was able to sell my 450D kit for more than I bought it a year ago, coz the price for that camera has climbed considerably since then, whereas the price of the 50D has almost halved (largely, I think, because of the launch of the 500D, which, on paper at least, seems to be a match for the 50D, but in practice is quite different). I also bought mine mint 2nd hand; there's a lot floating about in that market coz of the recent launch of the 7D. As a result, I needed only to make a small outlay for the new camera, and I got so much new camera for the money! It really has amazing IQ and is very speedy; you may well have read about the high ISO/noise issues due to it's densely pixelated sensor, but I'm having none of that! (for a useful debate about this check out
http://www.talkphotography.co.uk/forums/showthread.php?t=179054)
Of course, you could always upgrade to a 40D for less, but the 50D's IQ is just as good, better for cropping, and not least it's newer and has a better feature set.
If you can get it for around £700 new, or less
second hand, I'd say go for it; you'll see a vast improvement over your current camera.
Cheers