Well you can get a External HDD(EHDD) if you don't already have one.
Simply partition a 32GB space of your EHDD,then format the 32GB(partition) of the external HDD to fat32 and then plug
the EHDD into your PS3.The PS3 will recognize the 32GB of space when doing a backup(utility),
then performe a system backup to the EHDD.Then get your new GM MGS4 bundle and do a system restore.
Restore the backup you just did to your EHDD and thats it.Your,GM MGS4 will be the exact same PS3 as you orginal 40Gb.
Plus ,now you will have a backup of your original game save if ever you upgrade your HDD or your PS3 crashes. One more thing if you need more than 32GB to perform a back up simply create another 32gb partition and format that to fat32 also.then you should have enough space.