Getting an SLR has completely ruined my sense of composition. I am slowly working my way back, but its just a LOT easier to compose with a ~3" LCD screen showing you almost exactly what you're going to get.

Also, the Xsi's viewfinder isn't the greatest in the world.

I would get the 50mm f/1.8 to get a feel for what the camera can do in low light with no flash, with a (relatively) 'fast' lens.... but it depends on what you take photos of mostly.

You might get a lot more use out buying a 55-250mm IS f/4-5.6 zoom lens instead ($250 new, ~$200 used). Its a good budget telephoto lens (Canon started making it to compete with Nikon's 50-200 VR lens). It is light, compact and takes pretty decent photos.... just don't try to shoot action with it unless its a very, very sunny day :-) (not very fast at all!).