just ordered my 3rd off amazon yesterday (along with a new camera body - upgrading my xti to a t1i 🙂 )
adding a sigma 30mm 1.4
have a 50mm 1.8 and the 17-55 IS kit lens.
I pretty much only shoot in low light.
the 50mm came with the camera (bought it off a friend). the 50 can be a little tight on a cropped camera when trying to shoot party or bar pics. I think the 30mm will be a better fit and better focusing in low light, plus the 1.4 is a nice little bump over the 1.8.
i'll keep the 50mm because it's so light, and might be better in some instances.