For a "poor college student" your 18-55 is good, as are the recommended 55-250 and 85 f/1.8. Do note that these zoom lenses are generally at their sharpest nearer the middle of their zoom range and at f/8 or f/11. The 85 is a great lens, fast and accurate to focus and very sharp even wide open. The XTi is a good camera, but above ISO 400 it's files tend to get pixelated. Try Tv mode, ISO 400, spot metering, shutter 1/800sec or higher if there's enough light. Don't get discouraged if only the dog at the center of the picture is fully focused, since high shutter speed requires wide aperture and thus shallow depth of field.
I agree with wcpeabody, test the limits of your current equipment before spending money on another lens.