> thanks for your help!
Currently the largest games are 64M, so that should do fine if you are on a budget. The advantage to having a 128M or 256M cart is that you can put multiple games onto a single cart so you don't have to flash it every time you want to play a new game. There is also the possibility that future games could exceed 64M.
Personally I think the 256M cart is a good deal since the price difference isn't that much from a 128M cart and it includes EEPROM support. If you don't mind patching games and not planning on playing anything large though, 64M should do fine.