The lens are a more important investment than the camera body, you will most likely be replacing the camera body well before you replace the lens. A fast lens is always good, they offer the best performance, fastest autofocus, and the brightest viewfinder. A "fast" lens is something in the range of f2.8 to f4 on a zoom, and f1.4 to f2.8 on a fixed lens. The prime (fixed) 50mm f1.8 is always a good investment, its cheap, fast, and performs great. But as frog said, it also depends on what you plan on shooting that determines what range of lens you'd like. On the XTI's crop sensor, 10-30mm is a good range for landscape shots, 25-100 is good for portraits, 70-200/300 is good for sports and wildlife, and 200-400 is good for bird photography, for example.

Another important thing to consider is what kind of, if any, macro photography you'd like to do. Macro is close-up photography, and various lenses offer various distances for macro, and the relative closeness is measured by the 'magnification'. A 1:4 magnification ratio is a mild macro, 1:2 is closer, and 1:1 is a seriously close macro (you can shoot grains of rice, get real close on insects, etc). A 1:1 magnification simply means that the image is exactly identical in size to the sensor, say you are shooting a grain of rice - the grain of rice will be the same size on the sensor as it is in the field. Also keep in mind, when you see say a 70-200 lens that has a 1:2 magnification, that magnification can only be achieved at the telephoto end of the lens, as focus distance is the same across the entire zoom range.

Some popular ones that are always worth a look is the 24-105 L, the 70-200 L (f4 or f2.8, and/or IS, depending on budget), and the 50mm (f1.8 or f1.4, again depending on budget). However, if you want a lens more specialized for a particular type of shooting, you may want to also consider other options. Let us know what kind of shooting you would like to do, and what kind of budget you are alloting for lens - and we can show you some good lens options.