Backing up your GT5 progress is pretty easy and doesn't require too much space.
Under Games on the XMB enter the PS3 Game Save Data menu and find the GT5 Game save data and press triangle to bring up the options and select copy - follow the prompts to copy to your USB flash drive or whatever you are backing up to.
If you want to backup your entire PS3 system you will need a big enough external HD (FAT32 formatted) - this can take ages but you have a complete image of your system and can restore everything to a new system or HD if you need to - including all downloads, game updates etc...