Just to clarify, you do know your 2004 account is now going to be a RS3 account, right?
Also; if you ever had Membership before and can prove with card/name/etc. that you were the one who bought it that'll help you.
ISP is good too, creation of year is nice too but month would be better. I could easily just guess years until they approved so month and date are very important, recovery questions, birthday, email, etc.
Basically anything and everything you know about the account and I mean ANYTHING.