To TS, many other posts about this comparison already. There have been mixed reviews about the noise levels. I think what's more important is your budget. I mentioned this before that its not just the extra $400-$500 you have to consider, but extra lense cost. An 18-55 kit can work very well with the 40D, and I'm not saying it can't on the 50D, but just that you get the best out of the 50D with better lenses, and the starting point would definitely be the 18-200. So with the extra cost from the body and lense, it totals to quite abit. Something to consider.
Do let us know what your budget is so we can advise you accordingly.
I purchased the 50D with the 18-200mm kit lens.
I eventually got rid of the lens and I'm now planning to get the EF 70-200 f2.8L.
If low light is your concern, then I recommend you getting maybe just the 40/50D body with the EF-S 17-55 f2.8 lens. That lens will be a real keeper.
The thing with the 50D is that you need to use the better lenses to take full advantage of it.
If you put a bad lens, the image will be even worse than the same lens on a 40D.
This is due to the high pixel density of the 50D.
So in this case, your budget should take into account what lenses you plan on getting in the future.
If you have the budget to get a few L lenses, go for the 50D.
Otherwise the 40D is a better choice.
As for direct comparison of low-ISO performance for the 40D and 50D, you can have a look at reviews around for more details.
I believe (read somewhere) that 50D has slightly better ISO performance.
They employ what they call "gap-less" micro-sensor.
in some way by naming it 50D it will definitely be seen as the replacement to 40d. many will disagree though saying it's just a sister to the 40D. but the thing is, it doesn't really matter if 50D will replace 40D or not, because Canon broke the 18 month cycle with the 50D, so we won't know if the 60D will come out soon to replace the 50D, and so on and so forth ...
bottom line is, there will always be a better model in the works or coming out soon, and prices will keep changing, and no one can afford to wait or we'll keep waiting forever