why are you upgrading?
The 40d is essentially the same quality image, the bodies better built with more options but image wise you shouldn't really see any difference, I'd go for the 50d in your case if money allows.
Having pretty much exactly same kit as in your sig, I have to say that while I have regretted not going for the 50D in the first place, mainly from a handling point of view (I've got a bunch of 'nana's for fingers and the 450D is a bit fiddly i comparison to the 40/50D) but from an Image Quality point of view I reckon that money spent on better glass would be my first point of call. Then again, I don't shoot motorsport or anything that needs a faster frame rate... landscapes aren't noted for running away from the 'togger.
Pro's:
Better handling (i.e bigger)
Better noise handling in higher ISO
6.5 FPS (can burst 17 in comparison to 6 on the 450D).
Con's
Drop in megapixel.
I agree with getting some L glass before upgrading the body.
Unless of course you were planning on selling the kit lens with your 450D and intended to buy a good lens with the new body.
I really like my 55-250mm IS lens and I'd be curious to know what the shots are like with this lens on a 50D.
Be curious no more.
http://www.flickr.com/search/?ss=2&w=all&q=55-250+50d&m=text
Flickr link with the search parameters of the 55-250mm Is and 50D.
Tommy.
So would I see a real difference with the 50D?