First of all its called CSSU, not CUSS.

If you want to install CSSU and/or would like to have slightly longer battery life (it heavily depends on how you use your device actually) you should install PR1.3 instead of PR1.2

I am guessing your device will not boot is because of missing kernel modules. Lets say when you made your backup via backupmenu, you had kernel-power47 installed. However sometime afterwards you have upgraded that kernel-power47 to kernel-power48. Now backupmenu cannot backup nor restore the actual kernel because the actual kernel sits in a location not accessible by a regular filesystem (i.e. most likely ubifs based as opposed to regular block devices). So when you restored your backup image which only had kernel-power47 kernel modules instead of kernel-power48 kernel modules, your device reboots, loads up kernel-power48 and it goes into kernel panic declaring kernel-power48 modules are not found.

As for the flashing technique, yes those are the correct files but you will lose backupmenu along with many other things that sits in rootfs. Nothing in /opt, /home, or /home/user/MyDocs will be lost when you flash using the combined (aka fiasco) image.

For a guide on flashing read here: http://wiki.maemo.org/Updating_the_t...ng_the_Flasher

If you have windows 64bit, you might want to have a look at this: http://wiki.maemo.org/Updating_the_t...nd_Workarounds