It looks like the XSi may be the way for you to go. If you're looking to buy new, it won't be a problem to get the body/kit lens set for under $600. Tack on about $150 more and you get both the 18-55 and 55-250, which will most definitely get you going.
If you think you'll want to add the heavier lenses later on, definitely get the grip. My dad has the T1i and some heavy lenses and it definitely helps balance them better. Of course, the grip can wait until you get ready to start adding heavier lenses, unless you've got bigger hands like my dad does and find the grip makes the camera more comfortable to hold, period.
I can definitely appreciate the lens upgrade suggestions, but if a more expensive lens would put too much of a strain on your budget, then just stick with the kit lenses. What is most important is that you just start somewhere. You can always upgrade to better glass later on.