To answer your upgrade question: yes, you must buy TBC to get WotLK to work (I think that was your question)

To explain Oatboat's comment: using your brothers account is against the rules and can result in BOTH your accounts being banned. There is no way around it, refer yourself two new accounts and ditch your brothers account.

As for system specs, I had a computer of nearly identical specs that couldn't even dual box in Org. Expect your machine to blow up if you three-box in Dalaran.