You will need a blackberry specific data plan (blackberry data service) to get the push email. You may be able to use your carrier's generic cell phone data service to access web-based email using opera mini, if you cannot purchase blackberry service. In that case, see blackberryfaq.com and the article on using a generic data plan. Good luck.
Anyway, other applications like Yahoo Messenger or things like this will work?
Also, I didn't understand from FAQ if I use Opera Mini to read my Gmail email, will I be able to view the attachments?