Depends on what you want to do.
RaBit is good for changing basic configuration of cards, memory size, Vivo options, default speeds, etc... And for hardcore coders a Hex editor is best, change config flags, substrate fuse data, whole ident strings.
To be honest there are some good people who will do your BIOS for you over at Techpowerup. And will help answer questions, and proveide better insite to modding you card.