Replying to Canon Lense Help

Hey guys, heres a little background info: im shooting with a digital rebel XTi using the kit lense (18-55 f3.5-5.6). Im looking to buy a wide angle or fisheye lense for it, preferabley below or near 500-600$. I shoot portraits,concerts, nature shots, stuff like that. I wnat a lense that can add a different feel to my photos, so that ill know that im not still shooting with my standard lense. Any recomendations? I was thinking about the Canon 10-22mm f3.5-4.5 or the Sigma equivalent. I want something i can shoot in low light too.