If you have a or an early PSP-2000 (non-v3) , you can make the firmware permanent later.
The most common mistake users make is downloading 660.PBP from Sony and running it immediately. If you run the 6.60 official updater on 3.52 M33, you risk a "brick" (a dead, black-screened PSP). Why? Because 3.52 M33 lacks the required IPL (Initial Program Loader) updates that happened in versions 3.71 and 4.01. psp 352 m33 upgrade to 660 portable