You basically see what you get. Only the Canon 1D series actually show 100% of the image through the view finder I believe, others show a few percent less. Why that is is unclear to me (must be expensive to make it that precise) but basically in every DSLR you see through the view finder what will be on the photo, no matter what kind of lens you mount on it.sorry I missed the lens war, but you may consider the G7 or a lumix SLR-like model, as they're closer to an XTi, but have all the glass you need built in and will not hit so hard on your pocket.
EDIT: this thread just made me think of an interesting question. when using a "35mm" lens on an APS-C DSLR, is the image in the viewfinder what is being taken by the sensor, or does the viewfinder show more than the final taken photo? hope I phrased that well...