i have used both the xt and xti extensively plus a plathora of lenses. my suggestion would be as follows:
get the xti body and get the tamron 17-50mm f2.8 lens. i am using this same combination and my photos come out clear, sharp and crisp without a whole lot of effort. the 28-135 is a good lens but its not a great lens. for a wide range of reviews on the lens...i suggest you go to
fredmiranda.com: Specialized in digital cameras, photoshop actions, and digital darkroom
i also have the the nifty fifty, plastic fantastic or whatever else is called...its a great little lens but being a prime, its use is and will always be limited. you can always get the 50 later if your budget doesnt allow right now.
at the end of the day, its not the gear..its the photographer so even the fastest lens will not make you a pro but a fast lens does makes it easy to take sharp and clear photos if your technique is right. f2.8 will easily allow you to take photos in indoor light...even outside when the light is low handheld.