Yep!
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 It has been replaced by the BE-E2N, which supposedly has better sealing around the battery compartment.
yes
All canon EF and EFS lenses fit on all canon dslr's
Any standard hot-shoe flash should fit the XSi, though check the flash sync voltage to be on the safe side. Using a Canon EX-series Speedlite will give you a variety of additional features; other flashes won't be able to synchronise at shutter speeds faster than 1/200th sec, and won't work in live view mode, either, for example.
Any canon ef or efs lenses will fit on all of canon's dslr
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.