I would avoid the D60, I had a bad experience with one personally and had to return it (many dead pixels on the sensor). A friend of mine purchased a D40, and loves it, though he's running into limitations on usable lenses, which you'll hit with the D60 as well - neither the D40 or D60 can autofocus with Nikon's "AF" lenses, you need to have "AF-S" lenses in order to autofocus (if you're fine manually focusing though, this is a moot point).
I purchased a Rebel XSi after returning my D60, and love it. It's a great camera, and from what I understand, the XS is very similar. My only gripe so far is the relatively few AF points (especially when compared to the Nikon D90 or D300).
As for lenses, no, IS means "Image Stabilization", while USM means "Ultra Sonic Motor" - IS lenses will help negative some of the effects of shakey hands while you take pictures; USM lenses just tend to autofocus quicker and quieter. Of the two Canon lenses you listed, I'd suggest the 55-250mm IS lens - it's fantastic.