You can't brick your phone with an update in itunes because quickpwn will unlock/activate your original iPhone. and on the 3G it doesn't change anything other than activation.
Remmeber you cannot update using custom firmware only restore.
So if you want to keep all your data the best option is use itunes to update then quickpwn to unlock/activate/jailbreak.
But be aware that your iPhone may be relocked and non-activated when it restarts after the update (it depends on if apple updated the modem on the 2.1 update which I am not sure about!).