Yes, the BG-E2 battery grip will work on the Canon 40D, 20D, and 30D. It has been replaced by the BE-E2N, which supposedly has better sealing around the battery compartment.
Sorry - it only fits XSi, XS and T1i.
All canon EF and EFS lenses fit on all canon dslr's
No, as with most batteries this one is only compatible with certain canon camcorder models. There are too many supported models to list here (which may be a good thing) but any product information page for this battery will tell you what models are supported.
Any canon ef or efs lenses will fit on all of canon's dslr
Yep!
Yep.
No, Canon lenses do not fit Nikon cameras. Canon lenses are designed for Canon cameras and use a different lens mount system than Nikon cameras.
No, a Canon lens will not fit on a Nikon camera because they have different lens mounts.
No
Canon NB-4L fits the CANON Powershot SD200 and SD300.
No, a Canon lens cannot fit a Nikon camera because they have different lens mounts.
The Canon EF lens will only fit Canon SLR camera bodies, unfortunately.