"Notification" should be enabled if u wan the sms sound when receiving sms using handcent. but if u dun wan the message icon to show at the status bar, u can set it separately in the "Notification Setting" (something called privacy mode) .

actually u can upgrade but cannot downgrade if you tied to the contract. (u need to go Maxi$ center for doing so).
However, before u do that, make sure ur data usage is not exceeding what so called "prorated" limit. Means if day 1 u registered for 500mb, and day 10 u wish to upgrade, the total data usage must not exceed 500mb/30*10days = 166.667mb. otherwise, everything that exceed this limit will count as per use instead of package. (which cost u 0.05sen/kb)

thus, choose wisely