30D better cropping

I have the 30D and I like to shoot birds. I use L glass- 500mm, 100-400, and 70-200 f4 IS. If I could compare 25% to 50% crops from my 30D to a 40D or 50D with one of these lenses, how would you describe the IQ of these crops? Slight, noticeable, significant, fantastic. I hope some have upgraded to 40D and 50D and can compare.

I try not to use higher than ISO 400, or perhaps 640. I hate noise! How much difference in noise between these? Thanks.