Hey milefile!
Here is the breakdown.
You will need to purchase a "Shark Port" in order to save your memory card files to your PC. You can usually get one at your local electronics store or you can order one from
www.gameshark.com
With a Shark Port, just like Slip2rock mentioned, you will be able to save your memory card to your PC, and you will be able to e-mail your replays. That comes in handy when you want to compete in a online racing challenge.
The Shark Port will also enable you to import other game saves and "cheat codes" that you can download onto your memory card.
With this feature you will be able to download a hybriding program. This program let's you change around your cars with various other car parts.
Here is a link to that program.
http://63.169.100.88/uploads/GT3/Hacking Programs/GT3gEditv22.zip
Further more, a "Game Shark" will only allow you to use and create your own "cheat codes." It will not let you save any info or files onto your PC.
The Game Shark will allow you to hybrid vehicles also, but not in the same easy format that the MK Program has.
Here is a link to a web site that contains all the info you will need to have in regards to hybriding a car.
http://63.169.100.88/cgi-bin/ubb-cgi/ultimatebb.cgi
I hope that this has helped you out.
Peace,
Boom