If you want to upgrade to iOS4, then you shouldn't brick the phone. You will lose the jailbreak. As far as I know, upgrading to the newest iTunes shouldn't hurt. But don't quote me on that!!! After iTunes 9.2 came out right before iOS4 was released, JB people said that the JB info & iTunes weren't playing nicely together. I saw a new 9.2 specific file that had to be used for JB phones (Spirit) with 9.2.
I guess the short answer is, if you love your jailbreak, don't upgrade.