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 It has been replaced by the BE-E2N, which supposedly has better sealing around the battery compartment.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 NB-4L fits the CANON Powershot SD200 and SD300.
The Canon EF lens will only fit Canon SLR camera bodies, unfortunately.
The Canon AE1 lens will not fit a Canon digital. You can buy an adapter that will allow the lens to work in manual focus mode on the digital camera.
Sorry No. and no one produces an adaptor to convert the praktica lenses to canon fit either.
The cartridges that fit the Canon 17 55 are the PG-210XL ink cartridge, as well as the CL-211 XL cartridge, both manufactured and sold officially by Canon.