If you are using it for just game saves then you don't need more than 32mb. However, I'd recommend you get a 1gb stick for if/when you decide you want to put on music/movies/etc. Plus there will be the occasional demo you might want to download so I would recommend a 1gb+.
Thanks for the reply, I just wasn't sure how much memory game saves could actually take up. I'd probably get a bigger card if sony ever allows ps1 downloads straight to the psp, or I can find someone with ps3 that will let me download the games.