Do you have an external HDD in FAT32?
That's the easiest solution.
In 320GB's multiMAN, highlight the game you want to transfer, press triangle and go to options. Select "copy" (or transfer), it will asks if you want to copy to /dev_usb0/ say Yes.
If the games has big files, they will be split in multiple 4GB files.
Now, connect the external HDD to the 80GB's multiMAN, and do the same "copy" command. It will asks if you want to copy the game to /dev_hdd0/ say yes.
If the games has split files, they will be joined automatically.
That's only for the game itself. then you need to let the game installs the game data again.
If the source game has updates with patches, you may want to copy them too instead of letting the network install it (located in /dev_hdd0/game/GAMEID/ folder*), or repatch the game with pkg if required. (it depend on the cfw version).
*GameID :
This folder change for each game.
About Save files and trophies, there's a way to convert the owner, but it doesn't work very good.
You need a tool, and a save for that game from each owner. it will replace the data in the target file. (or replace the owner's info in the source file)
For trophies, there are trophies editors but I don't remember if you can select one by one or if it unlock everything at once.
****
If you don't have an external HDD you'll need a way to list both PS3 content on a single console to initiate the transfer.
I don't think it's possible with multiMAN file browser.
FTP could be used, but you need a client working in FXP (remote server to remote server), which require a computer. I don't know PS3 homebrew working as both client and server at the same time.