It's just that that is an awful large list of things that can easily be answered by yourself by going to the tutorials page http://www.xbox-scene.com/tutorials.php
But because I'm a VERY nice guy, allow me to assist you:
play all video formats and all dvds (well one that will let me put xbox media player on?)
The Chip has nothing to do with it, it is the software on the Xbox that controls this. Any chip will allow you to run the application necessary to do this though.
let me play back-up games
Yes, they all will.
is there one that i can play roms on????
Refer to the answer I gave for your first question.
is there one that can link up to the computer?
Most do, but the 4th gen chips (Xenium/SmartXX/XBIT/X3) have more ways of doing so. the newer chips have network capabilities (except the XBIT which utilizes USB) which make transfering BIOS files faster and easier then ever.
and maybe back up games on hard drive (i know i need a upgrade on hard drive)
Again, refer to the first answer I gave for your first question.
Anymore questions compadre?