I have the rebel xti and I just picked up a Sigma 10mm Fisheye, it's fantastic. I would post photos taken with it, but I'm at school, hah. I would check it out, it's a great lense. And amazing quality even up to 1600 ISO.
The lens does not affect the quality of the picture when you're talking about the ISO. ISO is only in relation to your sensor... anyway. Lenses, can be sharp, bright and with well constructed... not good on every ISO setting...