First you need to backup everything on your PS3 excluding trophies (can be synced to PSN)
About 1Tb HDD, i don't recommend that as the PS3 Fat will become slower than usual so i advise you to install 500Gbs HDD.
Now about backing up Trophies:
Inside multi man go to file manager/root/dev_HDD0/home/000000X(depending on your profile number)/& copy trophy data
to USB or External HDD. after upgrading HDD,restoring back up & installing multiman go to the same directory above & copy your trophy folder there. now you need to access recovery mode:
1. Turn off your PS3
2. Hold the power button (you'll hear a beep) keep holding until the PS3 shuts down.
3. Press & hold again (you'll hear a beep) & then two consecutive beeps,take off your finger.
now you accessed recovery mode
choose Rebuild data base i think its the third option & your trophies will be restored.
now here is a good guide i found on how to install a new HDD:
Have a nice day.