Comparing noise between cameras

I’ve started to shoot football (soccer)and have quickly come to the realisation that my 400d isn’t going to cope very well with evening games (in winter with afternoon games either). With that in mind I’m on the verge of upgrading to a 7d (although I could still be persuaded by the 50d or 60d).

However I just wanted to know if there was any specific way I could compare the noise levels at different iso ranges? I read somewhere that the noise levels in the digic 4 processor at iso 6400 were similar to iso 1600 in the digic 3.

However, I can’t find any comparison between the digic 4 (or 3) and the digic 2 (in the 400d) and I guess I’d just like to get an idea of what noise I could expect at 6400 compared to what I’m used to.

Thanks for any help!

Also, if it helps, I’d primarily be using a 70-200 f/4 and would like to keep using that as it’s so much lighter than the 2.8.

Go here:

http://www.dpreview.co​m/

Although I don't think they offer a direct comparison between the XTI and the 7d, you can compare 4 different cameras against each other, just pick a camera from the reviews, and choose the "compared to" link from the drop-down menu at the top.

One web site is well known for extensive comparisons of cameras' ISO performance.